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Charles Darwin: The origin of life
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CHAPTER XIV.
MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY -- EMBRYOLOGY --
RUDIMENTARY ORGANS.
Classification, groups subordinate to groups -- Natural system -- Rules and
difficulties in classification, explained on the theory of descent with
modification -- Classification of varieties -- Descent always used in
classification -- Analogical or adaptive characters -- Affinities, general,
complex and radiating -- Extinction separates and defines groups --
Morphology, between members of the same class, between parts of the same
individual -- Embryology, laws of, explained by variations not supervening
at an early age, and being inherited at a corresponding age -- Rudimentary
organs; their origin explained -- Summary.
CLASSIFICATION.
>From the most remote period in the history of the world organic beings have
been found to resemble each other in descending degrees, so that they can
be classed in groups under groups. This classification is not arbitrary
like the grouping of the stars in constellations. The existence of groups
would have been of simple significance, if one group had been exclusively
fitted to inhabit the land, and another the water; one to feed on flesh,
another on vegetable matter, and so on; but the case is widely different,
for it is notorious how commonly members of even the same subgroup have
different habits. In the second and fourth chapters, on Variation and on
Natural Selection, I have attempted to show that within each country it is
the widely ranging, the much diffused and common, that is the dominant
species, belonging to the larger genera in each class, which vary most.
The varieties, or incipient species, thus produced, ultimately become
converted into new and distinct species; and these, on the principle of
inheritance, tend to produce other new and dominant species. Consequently
the groups which are now large, and which generally include many dominant
species, tend to go on increasing in size. I further attempted to show
that from the varying descendants of each species trying to occupy as many
and as different places as possible in the economy of nature, they
constantly tend to diverge in character. This latter conclusion is
supported by observing the great diversity of forms, which, in any small
area, come into the closest competition, and by certain facts in
naturalisation.
I attempted also to show that there is a steady tendency in the forms which
are increasing in number and diverging in character, to supplant and
exterminate the preceding, less divergent and less improved forms. I
request the reader to turn to the diagram illustrating the action, as
formerly explained, of these several principles; and he will see that the
inevitable result is, that the modified descendants proceeding from one
progenitor become broken up into groups subordinate to groups. In the
diagram each letter on the uppermost line may represent a genus including
several species; and the whole of the genera along this upper line form
together one class, for all are descended from one ancient parent, and,
consequently, have inherited something in common. But the three genera on
the left hand have, on this same principle, much in common, and form a
subfamily, distinct from that containing the next two genera on the right
hand, which diverged from a common parent at the fifth stage of descent.
These five genera have also much in common, though less than when grouped
in subfamilies; and they form a family distinct from that containing the
three genera still further to the right hand, which diverged at an earlier
period. And all these genera, descended from (A), form an order distinct
from the genera descended from (I). So that we here have many species
descended from a single progenitor grouped into genera; and the genera into
subfamilies, families and orders, all under one great class. The grand
fact of the natural subordination of organic beings in groups under groups,
which, from its familiarity, does not always sufficiently strike us, is in
my judgment thus explained. No doubt organic beings, like all other
objects, can be classed in many ways, either artificially by single
characters, or more naturally by a number of characters. We know, for
instance, that minerals and the elemental substances can be thus arranged.
In this case there is of course no relation to genealogical succession, and
no cause can at present be assigned for their falling into groups. But
with organic beings the case is different, and the view above given accords
with their natural arrangement in group under group; and no other
explanation has ever been attempted.
Naturalists, as we have seen, try to arrange the species, genera and
families in each class, on what is called the Natural System. But what is
meant by this system? Some authors look at it merely as a scheme for
arranging together those living objects which are most alike, and for
separating those which are most unlike; or as an artificial method of
enunciating, as briefly as possible, general propositions--that is, by one
sentence to give the characters common, for instance, to all mammals, by
another those common to all carnivora, by another those common to the
dog-genus, and then, by adding a single sentence, a full description is
given of each kind of dog. The ingenuity and utility of this system are
indisputable. But many naturalists think that something more is meant by
the Natural System; they believe that it reveals the plan of the Creator;
but unless it be specified whether order in time or space, or both, or what
else is meant by the plan of the Creator, it seems to me that nothing is
thus added to our knowledge. Expressions such as that famous one by
Linnaeus, which we often meet with in a more or less concealed form,
namely, that the characters do not make the genus, but that the genus gives
the characters, seem to imply that some deeper bond is included in our
classifications than mere resemblance. I believe that this is the case,
and that community of descent--the one known cause of close similarity in
organic beings--is the bond, which, though observed by various degrees of
modification, is partially revealed to us by our classifications.
Let us now consider the rules followed in classification, and the
difficulties which are encountered on the view that classification either
gives some unknown plan of creation, or is simply a scheme for enunciating
general propositions and of placing together the forms most like each
other. It might have been thought (and was in ancient times thought) that
those parts of the structure which determined the habits of life, and the
general place of each being in the economy of nature, would be of very high
importance in classification. Nothing can be more false. No one regards
the external similarity of a mouse to a shrew, of a dugong to a whale, of a
whale to a fish, as of any importance. These resemblances, though so
intimately connected with the whole life of the being, are ranked as merely
"adaptive or analogical characters;" but to the consideration of these
resemblances we shall recur. It may even be given as a general rule, that
the less any part of the organisation is concerned with special habits, the
more important it becomes for classification. As an instance: Owen, in
speaking of the dugong, says, "The generative organs, being those which are
most remotely related to the habits and food of an animal, I have always
regarded as affording very clear indications of its true affinities. We
are least likely in the modifications of these organs to mistake a merely
adaptive for an essential character." With plants how remarkable it is
that the organs of vegetation, on which their nutrition and life depend,
are of little signification; whereas the organs of reproduction, with their
product the seed and embryo, are of paramount importance! So again, in
formerly discussing certain morphological characters which are not
functionally important, we have seen that they are often of the highest
service in classification. This depends on their constancy throughout many
allied groups; and their constancy chiefly depends on any slight deviations
not having been preserved and accumulated by natural selection, which acts
only on serviceable characters.
That the mere physiological importance of an organ does not determine its
classificatory value, is almost proved by the fact, that in allied groups,
in which the same organ, as we have every reason to suppose, has nearly the
same physiological value, its classificatory value is widely different. No
naturalist can have worked at any group without being struck with this
fact; and it has been fully acknowledged in the writings of almost every
author. It will suffice to quote the highest authority, Robert Brown, who,
in speaking of certain organs in the Proteaceae, says their generic
importance, "like that of all their parts, not only in this, but, as I
apprehend in every natural family, is very unequal, and in some cases seems
to be entirely lost." Again, in another work he says, the genera of the
Connaraceae "differ in having one or more ovaria, in the existence or
absence of albumen, in the imbricate or valvular aestivation. Any one of
these characters singly is frequently of more than generic importance,
though here even, when all taken together, they appear insufficient to
separate Cnestis from Connarus." To give an example among insects: in one
great division of the Hymenoptera, the antennae, as Westwood has remarked,
are most constant in structure; in another division they differ much, and
the differences are of quite subordinate value in classification; yet no
one will say that the antennae in these two divisions of the same order are
of unequal physiological importance. Any number of instances could be
given of the varying importance for classification of the same important
organ within the same group of beings.
Again, no one will say that rudimentary or atrophied organs are of high
physiological or vital importance; yet, undoubtedly, organs in this
condition are often of much value in classification. No one will dispute
that the rudimentary teeth in the upper jaws of young ruminants, and
certain rudimentary bones of the leg, are highly serviceable in exhibiting
the close affinity between Ruminants and Pachyderms. Robert Brown has
strongly insisted on the fact that the position of the rudimentary florets
is of the highest importance in the classification of the Grasses.
Numerous instances could be given of characters derived from parts which
must be considered of very trifling physiological importance, but which are
universally admitted as highly serviceable in the definition of whole
groups. For instance, whether or not there is an open passage from the
nostrils to the mouth, the only character, according to Owen, which
absolutely distinguishes fishes and reptiles--the inflection of the angle
of the lower jaw in Marsupials--the manner in which the wings of insects
are folded--mere colour in certain Algae--mere pubescence on parts of the
flower in grasses--the nature of the dermal covering, as hair or feathers,
in the Vertebrata. If the Ornithorhynchus had been covered with feathers
instead of hair, this external and trifling character would have been
considered by naturalists as an important aid in determining the degree of
affinity of this strange creature to birds.
The importance, for classification, of trifling characters, mainly depends
on their being correlated with many other characters of more or less
importance. The value indeed of an aggregate of characters is very evident
in natural history. Hence, as has often been remarked, a species may
depart from its allies in several characters, both of high physiological
importance, and of almost universal prevalence, and yet leave us in no
doubt where it should be ranked. Hence, also, it has been found that a
classification founded on any single character, however important that may
be, has always failed; for no part of the organisation is invariably
constant. The importance of an aggregate of characters, even when none are
important, alone explains the aphorism enunciated by Linnaeus, namely, that
the characters do not give the genus, but the genus gives the character;
for this seems founded on the appreciation of many trifling points of
resemblance, too slight to be defined. Certain plants, belonging to the
Malpighiaceae, bear perfect and degraded flowers; in the latter, as A. de
Jussieu has remarked, "The greater number of the characters proper to the
species, to the genus, to the family, to the class, disappear, and thus
laugh at our classification." When Aspicarpa produced in France, during
several years, only these degraded flowers, departing so wonderfully in a
number of the most important points of structure from the proper type of
the order, yet M. Richard sagaciously saw, as Jussieu observes, that this
genus should still be retained among the Malpighiaceae. This case well
illustrates the spirit of our classifications.
Practically, when naturalists are at work, they do not trouble themselves
about the physiological value of the characters which they use in defining
a group or in allocating any particular species. If they find a character
nearly uniform, and common to a great number of forms, and not common to
others, they use it as one of high value; if common to some lesser number,
they use it as of subordinate value. This principle has been broadly
confessed by some naturalists to be the true one; and by none more clearly
than by that excellent botanist, Aug. St. Hilaire. If several trifling
characters are always found in combination, though no apparent bond of
connexion can be discovered between them, especial value is set on them.
As in most groups of animals, important organs, such as those for
propelling the blood, or for aerating it, or those for propagating the
race, are found nearly uniform, they are considered as highly serviceable
in classification; but in some groups all these, the most important vital
organs, are found to offer characters of quite subordinate value. Thus, as
Fritz Muller has lately remarked, in the same group of crustaceans,
Cypridina is furnished with a heart, while in two closely allied genera,
namely Cypris and Cytherea, there is no such organ; one species of
Cypridina has well-developed branchiae, while another species is destitute
of them.
We can see why characters derived from the embryo should be of equal
importance with those derived from the adult, for a natural classification
of course includes all ages. But it is by no means obvious, on the
ordinary view, why the structure of the embryo should be more important for
this purpose than that of the adult, which alone plays its full part in the
economy of nature. Yet it has been strongly urged by those great
naturalists, Milne Edwards and Agassiz, that embryological characters are
the most important of all; and this doctrine has very generally been
admitted as true. Nevertheless, their importance has sometimes been
exaggerated, owing to the adaptive characters of larvae not having been
excluded; in order to show this, Fritz Muller arranged, by the aid of such
characters alone, the great class of crustaceans, and the arrangement did
not prove a natural one. But there can be no doubt that embryonic,
excluding larval characters, are of the highest value for classification,
not only with animals but with plants. Thus the main divisions of
flowering plants are founded on differences in the embryo--on the number
and position of the cotyledons, and on the mode of development of the
plumule and radicle. We shall immediately see why these characters possess
so high a value in classification, namely, from the natural system being
genealogical in its arrangement.
Our classifications are often plainly influenced by chains of affinities.
Nothing can be easier than to define a number of characters common to all
birds; but with crustaceans, any such definition has hitherto been found
impossible. There are crustaceans at the opposite ends of the series,
which have hardly a character in common; yet the species at both ends, from
being plainly allied to others, and these to others, and so onwards, can be
recognised as unequivocally belonging to this, and to no other class of the
Articulata.
Geographical distribution has often been used, though perhaps not quite
logically, in classification, more especially in very large groups of
closely allied forms. Temminck insists on the utility or even necessity of
this practice in certain groups of birds; and it has been followed by
several entomologists and botanists.
Finally, with respect to the comparative value of the various groups of
species, such as orders, suborders, families, subfamilies, and genera, they
seem to be, at least at present, almost arbitrary. Several of the best
botanists, such as Mr. Bentham and others, have strongly insisted on their
arbitrary value. Instances could be given among plants and insects, of a
group first ranked by practised naturalists as only a genus, and then
raised to the rank of a subfamily or family; and this has been done, not
because further research has detected important structural differences, at
first overlooked, but because numerous allied species, with slightly
different grades of difference, have been subsequently discovered.
All the foregoing rules and aids and difficulties in classification may be
explained, if I do not greatly deceive myself, on the view that the natural
system is founded on descent with modification--that the characters which
naturalists consider as showing true affinity between any two or more
species, are those which have been inherited from a common parent, all true
classification being genealogical--that community of descent is the hidden
bond which naturalists have been unconsciously seeking, and not some
unknown plan of creation, or the enunciation of general propositions, and
the mere putting together and separating objects more or less alike.
But I must explain my meaning more fully. I believe that the ARRANGEMENT
of the groups within each class, in due subordination and relation to each
other, must be strictly genealogical in order to be natural; but that the
AMOUNT of difference in the several branches or groups, though allied in
the same degree in blood to their common progenitor, may differ greatly,
being due to the different degrees of modification which they have
undergone; and this is expressed by the forms being ranked under different
genera, families, sections or orders. The reader will best understand what
is meant, if he will take the trouble to refer to the diagram in the fourth
chapter. We will suppose the letters A to L to represent allied genera
existing during the Silurian epoch, and descended from some still earlier
form. In three of these genera (A, F, and I) a species has transmitted
modified descendants to the present day, represented by the fifteen genera
(a14 to z14) on the uppermost horizontal line. Now, all these modified
descendants from a single species are related in blood or descent in the
same degree. They may metaphorically be called cousins to the same
millionth degree, yet they differ widely and in different degrees from each
other. The forms descended from A, now broken up into two or three
families, constitute a distinct order from those descended from I, also
broken up into two families. Nor can the existing species descended from A
be ranked in the same genus with the parent A, or those from I with parent
I. But the existing genus F14 may be supposed to have been but slightly
modified, and it will then rank with the parent genus F; just as some few
still living organisms belong to Silurian genera. So that the comparative
value of the differences between these organic beings, which are all
related to each other in the same degree in blood, has come to be widely
different. Nevertheless, their genealogical ARRANGEMENT remains strictly
true, not only at the present time, but at each successive period of
descent. All the modified descendants from A will have inherited something
in common from their common parent, as will all the descendants from I; so
will it be with each subordinate branch of descendants at each successive
stage. If, however, we suppose any descendant of A or of I to have become
so much modified as to have lost all traces of its parentage in this case,
its place in the natural system will be lost, as seems to have occurred
with some few existing organisms. All the descendants of the genus F,
along its whole line of descent, are supposed to have been but little
modified, and they form a single genus. But this genus, though much
isolated, will still occupy its proper intermediate position. The
representation of the groups as here given in the diagram on a flat
surface, is much too simple. The branches ought to have diverged in all
directions. If the names of the groups had been simply written down in a
linear series the representation would have been still less natural; and it
is notoriously not possible to represent in a series, on a flat surface,
the affinities which we discover in nature among the beings of the same
group. Thus, the natural system is genealogical in its arrangement, like a
pedigree. But the amount of modification which the different groups have
undergone has to be expressed by ranking them under different so-called
genera, subfamilies, families, sections, orders, and classes.
It may be worth while to illustrate this view of classification, by taking
the case of languages. If we possessed a perfect pedigree of mankind, a
genealogical arrangement of the races of man would afford the best
classification of the various languages now spoken throughout the world;
and if all extinct languages, and all intermediate and slowly changing
dialects, were to be included, such an arrangement would be the only
possible one. Yet it might be that some ancient languages had altered very
little and had given rise to few new languages, whilst others had altered
much owing to the spreading, isolation and state of civilisation of the
several co-descended races, and had thus given rise to many new dialects
and languages. The various degrees of difference between the languages of
the same stock would have to be expressed by groups subordinate to groups;
but the proper or even the only possible arrangement would still be
genealogical; and this would be strictly natural, as it would connect
together all languages, extinct and recent, by the closest affinities, and
would give the filiation and origin of each tongue.
In confirmation of this view, let us glance at the classification of
varieties, which are known or believed to be descended from a single
species. These are grouped under the species, with the subvarieties under
the varieties; and in some cases, as with the domestic pigeon, with several
other grades of difference. Nearly the same rules are followed as in
classifying species. Authors have insisted on the necessity of arranging
varieties on a natural instead of an artificial system; we are cautioned,
for instance, not to class two varieties of the pine-apple together, merely
because their fruit, though the most important part, happens to be nearly
identical; no one puts the Swedish and common turnip together, though the
esculent and thickened stems are so similar. Whatever part is found to be
most constant, is used in classing varieties: thus the great agriculturist
Marshall says the horns are very useful for this purpose with cattle,
because they are less variable than the shape or colour of the body, etc.;
whereas with sheep the horns are much less serviceable, because less
constant. In classing varieties, I apprehend that if we had a real
pedigree, a genealogical classification would be universally preferred; and
it has been attempted in some cases. For we might feel sure, whether there
had been more or less modification, that the principle of inheritance would
keep the forms together which were allied in the greatest number of points.
In tumbler pigeons, though some of the subvarieties differ in the important
character of the length of the beak, yet all are kept together from having
the common habit of tumbling; but the short-faced breed has nearly or quite
lost this habit; nevertheless, without any thought on the subject, these
tumblers are kept in the same group, because allied in blood and alike in
some other respects.
With species in a state of nature, every naturalist has in fact brought
descent into his classification; for he includes in his lowest grade, that
of species, the two sexes; and how enormously these sometimes differ in the
most important characters is known to every naturalist: scarcely a single
fact can be predicated in common of the adult males and hermaphrodites of
certain cirripedes, and yet no one dreams of separating them. As soon as
the three Orchidean forms, Monachanthus, Myanthus, and Catasetum, which had
previously been ranked as three distinct genera, were known to be sometimes
produced on the same plant, they were immediately considered as varieties;
and now I have been able to show that they are the male, female, and
hermaphrodite forms of the same species. The naturalist includes as one
species the various larval stages of the same individual, however much they
may differ from each other and from the adult; as well as the so-called
alternate generations of Steenstrup, which can only in a technical sense be
considered as the same individual. He includes monsters and varieties, not
from their partial resemblance to the parent-form, but because they are
descended from it.
As descent has universally been used in classing together the individuals
of the same species, though the males and females and larvae are sometimes
extremely different; and as it has been used in classing varieties which
have undergone a certain, and sometimes a considerable amount of
modification, may not this same element of descent have been unconsciously
used in grouping species under genera, and genera under higher groups, all
under the so-called natural system? I believe it has been unconsciously
used; and thus only can I understand the several rules and guides which
have been followed by our best systematists. As we have no written
pedigrees, we are forced to trace community of descent by resemblances of
any kind. Therefore, we choose those characters which are the least likely
to have been modified, in relation to the conditions of life to which each
species has been recently exposed. Rudimentary structures on this view are
as good as, or even sometimes better than other parts of the organisation.
We care not how trifling a character may be--let it be the mere inflection
of the angle of the jaw, the manner in which an insect's wing is folded,
whether the skin be covered by hair or feathers--if it prevail throughout
many and different species, especially those having very different habits
of life, it assumes high value; for we can account for its presence in so
many forms with such different habits, only by inheritance from a common
parent. We may err in this respect in regard to single points of
structure, but when several characters, let them be ever so trifling,
concur throughout a large group of beings having different habits, we may
feel almost sure, on the theory of descent, that these characters have been
inherited from a common ancestor; and we know that such aggregated
characters have especial value in classification.
We can understand why a species or a group of species may depart from its
allies, in several of its most important characteristics, and yet be safely
classed with them. This may be safely done, and is often done, as long as
a sufficient number of characters, let them be ever so unimportant, betrays
the hidden bond of community of descent. Let two forms have not a single
character in common, yet, if these extreme forms are connected together by
a chain of intermediate groups, we may at once infer their community of
descent, and we put them all into the same class. As we find organs of
high physiological importance--those which serve to preserve life under the
most diverse conditions of existence--are generally the most constant, we
attach especial value to them; but if these same organs, in another group
or section of a group, are found to differ much, we at once value them less
in our classification. We shall presently see why embryological characters
are of such high classificatory importance. Geographical distribution may
sometimes be brought usefully into play in classing large genera, because
all the species of the same genus, inhabiting any distinct and isolated
region, are in all probability descended from the same parents.
ANALOGICAL RESEMBLANCES.
We can understand, on the above views, the very important distinction
between real affinities and analogical or adaptive resemblances. Lamarck
first called attention to this subject, and he has been ably followed by
Macleay and others. The resemblance in the shape of the body and in the
fin-like anterior limbs between dugongs and whales, and between these two
orders of mammals and fishes, are analogical. So is the resemblance
between a mouse and a shrew-mouse (Sorex), which belong to different
orders; and the still closer resemblance, insisted on by Mr. Mivart,
between the mouse and a small marsupial animal (Antechinus) of Australia.
These latter resemblances may be accounted for, as it seems to me, by
adaptation for similarly active movements through thickets and herbage,
together with concealment from enemies.
Among insects there are innumerable instances; thus Linnaeus, misled by
external appearances, actually classed an homopterous insect as a moth. We
see something of the same kind even with our domestic varieties, as in the
strikingly similar shape of the body in the improved breeds of the Chinese
and common pig, which are descended from distinct species; and in the
similarly thickened stems of the common and specifically distinct Swedish
turnip. The resemblance between the greyhound and race-horse is hardly
more fanciful than the analogies which have been drawn by some authors
between widely different animals.
On the view of characters being of real importance for classification, only
in so far as they reveal descent, we can clearly understand why analogical
or adaptive characters, although of the utmost importance to the welfare of
the being, are almost valueless to the systematist. For animals, belonging
to two most distinct lines of descent, may have become adapted to similar
conditions, and thus have assumed a close external resemblance; but such
resemblances will not reveal--will rather tend to conceal their
blood-relationship. We can thus also understand the apparent paradox, that
the very same characters are analogical when one group is compared with
another, but give true affinities when the members of the same group are
compared together: thus the shape of the body and fin-like limbs are only
analogical when whales are compared with fishes, being adaptations in both
classes for swimming through the water; but between the the several
members of the whale family, the shape of the body and the fin-like limbs
offer characters exhibiting true affinity; for as these parts are so nearly
similar throughout the whole family, we cannot doubt that they have been
inherited from a common ancestor. So it is with fishes.
Numerous cases could be given of striking resemblances in quite distinct
beings between single parts or organs, which have been adapted for the same
functions. A good instance is afforded by the close resemblance of the
jaws of the dog and Tasmanian wolf or Thylacinus--animals which are widely
sundered in the natural system. But this resemblance is confined to
general appearance, as in the prominence of the canines, and in the cutting
shape of the molar teeth. For the teeth really differ much: thus the dog
has on each side of the upper jaw four pre-molars and only two molars;
while the Thylacinus has three pre-molars and four molars. The molars also
differ much in the two animals in relative size and structure. The adult
dentition is preceded by a widely different milk dentition. Any one may,
of course, deny that the teeth in either case have been adapted for tearing
flesh, through the natural selection of successive variations; but if this
be admitted in the one case, it is unintelligible to me that it should be
denied in the other. I am glad to find that so high an authority as
Professor Flower has come to this same conclusion.
The extraordinary cases given in a former chapter, of widely different
fishes possessing electric organs--of widely different insects possessing
luminous organs--and of orchids and asclepiads having pollen-masses with
viscid discs, come under this same head of analogical resemblances. But
these cases are so wonderful that they were introduced as difficulties or
objections to our theory. In all such cases some fundamental difference in
the growth or development of the parts, and generally in their matured
structure, can be detected. The end gained is the same, but the means,
though appearing superficially to be the same, are essentially different.
The principle formerly alluded to under the term of ANALOGICAL VARIATION
has probably in these cases often come into play; that is, the members of
the same class, although only distantly allied, have inherited so much in
common in their constitution, that they are apt to vary under similar
exciting causes in a similar manner; and this would obviously aid in the
acquirement through natural selection of parts or organs, strikingly like
each other, independently of their direct inheritance from a common
progenitor.
As species belonging to distinct classes have often been adapted by
successive slight modifications to live under nearly similar circumstances
--to inhabit, for instance, the three elements of land, air and water--we
can perhaps understand how it is that a numerical parallelism has sometimes
been observed between the subgroups of distinct classes. A naturalist,
struck with a parallelism of this nature, by arbitrarily raising or sinking
the value of the groups in several classes (and all our experience shows
that their valuation is as yet arbitrary), could easily extend the
parallelism over a wide range; and thus the septenary, quinary, quaternary
and ternary classifications have probably arisen.
There is another and curious class of cases in which close external
resemblance does not depend on adaptation to similar habits of life, but
has been gained for the sake of protection. I allude to the wonderful
manner in which certain butterflies imitate, as first described by Mr.
Bates, other and quite distinct species. This excellent observer has shown
that in some districts of South America, where, for instance, an Ithomia
abounds in gaudy swarms, another butterfly, namely, a Leptalis, is often
found mingled in the same flock; and the latter so closely resembles the
Ithomia in every shade and stripe of colour, and even in the shape of its
wings, that Mr. Bates, with his eyes sharpened by collecting during eleven
years, was, though always on his guard, continually deceived. When the
mockers and the mocked are caught and compared, they are found to be very
different in essential structure, and to belong not only to distinct
genera, but often to distinct families. Had this mimicry occurred in only
one or two instances, it might have been passed over as a strange
coincidence. But, if we proceed from a district where one Leptalis
imitates an Ithomia, another mocking and mocked species, belonging to the
same two genera, equally close in their resemblance, may be found.
Altogether no less than ten genera are enumerated, which include species
that imitate other butterflies. The mockers and mocked always inhabit the
same region; we never find an imitator living remote from the form which it
imitates. The mockers are almost invariably rare insects; the mocked in
almost every case abounds in swarms. In the same district in which a
species of Leptalis closely imitates an Ithomia, there are sometimes other
Lepidoptera mimicking the same Ithomia: so that in the same place, species
of three genera of butterflies and even a moth are found all closely
resembling a butterfly belonging to a fourth genus. It deserves especial
notice that many of the mimicking forms of the Leptalis, as well as of the
mimicked forms, can be shown by a graduated series to be merely varieties
of the same species; while others are undoubtedly distinct species. But
why, it may be asked, are certain forms treated as the mimicked and others
as the mimickers? Mr. Bates satisfactorily answers this question by
showing that the form which is imitated keeps the usual dress of the group
to which it belongs, while the counterfeiters have changed their dress and
do not resemble their nearest allies.
We are next led to enquire what reason can be assigned for certain
butterflies and moths so often assuming the dress of another and quite
distinct form; why, to the perplexity of naturalists, has nature
condescended to the tricks of the stage? Mr. Bates has, no doubt, hit on
the true explanation. The mocked forms, which always abound in numbers,
must habitually escape destruction to a large extent, otherwise they could
not exist in such swarms; and a large amount of evidence has now been
collected, showing that they are distasteful to birds and other insect-
devouring animals. The mocking forms, on the other hand, that inhabit the
same district, are comparatively rare, and belong to rare groups; hence,
they must suffer habitually from some danger, for otherwise, from the
number of eggs laid by all butterflies, they would in three or four
generations swarm over the whole country. Now if a member of one of these
persecuted and rare groups were to assume a dress so like that of a well-
protected species that it continually deceived the practised eyes of an
entomologist, it would often deceive predaceous birds and insects, and thus
often escape destruction. Mr. Bates may almost be said to have actually
witnessed the process by which the mimickers have come so closely to
resemble the mimicked; for he found that some of the forms of Leptalis
which mimic so many other butterflies, varied in an extreme degree. In one
district several varieties occurred, and of these one alone resembled, to a
certain extent, the common Ithomia of the same district. In another
district there were two or three varieties, one of which was much commoner
than the others, and this closely mocked another form of Ithomia. From
facts of this nature, Mr. Bates concludes that the Leptalis first varies;
and when a variety happens to resemble in some degree any common butterfly
inhabiting the same district, this variety, from its resemblance to a
flourishing and little persecuted kind, has a better chance of escaping
destruction from predaceous birds and insects, and is consequently oftener
preserved; "the less perfect degrees of resemblance being generation after
generation eliminated, and only the others left to propagate their kind."
So that here we have an excellent illustration of natural selection.
Messrs. Wallace and Trimen have likewise described several equally striking
cases of imitation in the Lepidoptera of the Malay Archipelago and Africa,
and with some other insects. Mr. Wallace has also detected one such case
with birds, but we have none with the larger quadrupeds. The much greater
frequency of imitation with insects than with other animals, is probably
the consequence of their small size; insects cannot defend themselves,
excepting indeed the kinds furnished with a sting, and I have never heard
of an instance of such kinds mocking other insects, though they are mocked;
insects cannot easily escape by flight from the larger animals which prey
on them; therefore, speaking metaphorically, they are reduced, like most
weak creatures, to trickery and dissimulation.
It should be observed that the process of imitation probably never
commenced between forms widely dissimilar in colour. But, starting with
species already somewhat like each other, the closest resemblance, if
beneficial, could readily be gained by the above means, and if the imitated
form was subsequently and gradually modified through any agency, the
imitating form would be led along the same track, and thus be altered to
almost any extent, so that it might ultimately assume an appearance or
colouring wholly unlike that of the other members of the family to which it
belonged. There is, however, some difficulty on this head, for it is
necessary to suppose in some cases that ancient members belonging to
several distinct groups, before they had diverged to their present extent,
accidentally resembled a member of another and protected group in a
sufficient degree to afford some slight protection, this having given the
basis for the subsequent acquisition of the most perfect resemblance.
ON THE NATURE OF THE AFFINITIES CONNECTING ORGANIC BEINGS.
As the modified descendants of dominant species, belonging to the larger
genera, tend to inherit the advantages which made the groups to which they
belong large and their parents dominant, they are almost sure to spread
widely, and to seize on more and more places in the economy of nature. The
larger and more dominant groups within each class thus tend to go on
increasing in size, and they consequently supplant many smaller and feebler
groups. Thus, we can account for the fact that all organisms, recent and
extinct, are included under a few great orders and under still fewer
classes. As showing how few the higher groups are in number, and how
widely they are spread throughout the world, the fact is striking that the
discovery of Australia has not added an insect belonging to a new class,
and that in the vegetable kingdom, as I learn from Dr. Hooker, it has added
only two or three families of small size.
In the chapter on geological succession I attempted to show, on the
principle of each group having generally diverged much in character during
the long-continued process of modification, how it is that the more ancient
forms of life often present characters in some degree intermediate between
existing groups. As some few of the old and intermediate forms having
transmitted to the present day descendants but little modified, these
constitute our so-called osculant or aberrant groups. The more aberrant
any form is, the greater must be the number of connecting forms which have
been exterminated and utterly lost. And we have evidence of aberrant
groups having suffered severely from extinction, for they are almost always
represented by extremely few species; and such species as do occur are
generally very distinct from each other, which again implies extinction.
The genera Ornithorhynchus and Lepidosiren, for example, would not have
been less aberrant had each been represented by a dozen species, instead of
as at present by a single one, or by two or three. We can, I think,
account for this fact only by looking at aberrant groups as forms which
have been conquered by more successful competitors, with a few members
still preserved under unusually favourable conditions.
Mr. Waterhouse has remarked that when a member belonging to one group of
animals exhibits an affinity to a quite distinct group, this affinity in
most cases is general and not special: thus, according to Mr. Waterhouse,
of all Rodents, the bizcacha is most nearly related to Marsupials; but in
the points in which it approaches this order, its relations are general,
that is, not to any one Marsupial species more than to another. As these
points of affinity are believed to be real and not merely adaptive, they
must be due in accordance with our view to inheritance from a common
progenitor. Therefore, we must suppose either that all Rodents, including
the bizcacha, branched off from some ancient Marsupial, which will
naturally have been more or less intermediate in character with respect to
all existing Marsupials; or that both Rodents and Marsupials branched off
from a common progenitor, and that both groups have since undergone much
modification in divergent directions. On either view we must suppose that
the bizcacha has retained, by inheritance, more of the character of its
ancient progenitor than have other Rodents; and therefore it will not be
specially related to any one existing Marsupial, but indirectly to all or
nearly all Marsupials, from having partially retained the character of
their common progenitor, or of some early member of the group. On the
other hand, of all Marsupials, as Mr. Waterhouse has remarked, the
Phascolomys resembles most nearly, not any one species, but the general
order of Rodents. In this case, however, it may be strongly suspected that
the resemblance is only analogical, owing to the Phascolomys having become
adapted to habits like those of a Rodent. The elder De Candolle has made
nearly similar observations on the general nature of the affinities of
distinct families of plants.
On the principle of the multiplication and gradual divergence in character
of the species descended from a common progenitor, together with their
retention by inheritance of some characters in common, we can understand
the excessively complex and radiating affinities by which all the members
of the same family or higher group are connected together. For the common
progenitor of a whole family, now broken up by extinction into distinct
groups and subgroups, will have transmitted some of its characters,
modified in various ways and degrees, to all the species; and they will
consequently be related to each other by circuitous lines of affinity of
various lengths (as may be seen in the diagram so often referred to),
mounting up through many predecessors. As it is difficult to show the
blood-relationship between the numerous kindred of any ancient and noble
family, even by the aid of a genealogical tree, and almost impossible to do
so without this aid, we can understand the extraordinary difficulty which
naturalists have experienced in describing, without the aid of a diagram,
the various affinities which they perceive between the many living and
extinct members of the same great natural class.
Extinction, as we have seen in the fourth chapter, has played an important
part in defining and widening the intervals between the several groups in
each class. We may thus account for the distinctness of whole classes from
each other--for instance, of birds from all other vertebrate animals--by
the belief that many ancient forms of life have been utterly lost, through
which the early progenitors of birds were formerly connected with the early
progenitors of the other and at that time less differentiated vertebrate
classes. There has been much less extinction of the forms of life which
once connected fishes with Batrachians. There has been still less within
some whole classes, for instance the Crustacea, for here the most
wonderfully diverse forms are still linked together by a long and only
partially broken chain of affinities. Extinction has only defined the
groups: it has by no means made them; for if every form which has ever
lived on this earth were suddenly to reappear, though it would be quite
impossible to give definitions by which each group could be distinguished,
still a natural classification, or at least a natural arrangement, would be
possible. We shall see this by turning to the diagram: the letters, A to
L, may represent eleven Silurian genera, some of which have produced large
groups of modified descendants, with every link in each branch and
sub-branch still alive; and the links not greater than those between
existing varieties. In this case it would be quite impossible to give
definitions by which the several members of the several groups could be
distinguished from their more immediate parents and descendants. Yet the
arrangement in the diagram would still hold good and would be natural; for,
on the principle of inheritance, all the forms descended, for instance from
A, would have something in common. In a tree we can distinguish this or
that branch, though at the actual fork the two unite and blend together.
We could not, as I have said, define the several groups; but we could pick
out types, or forms, representing most of the characters of each group,
whether large or small, and thus give a general idea of the value of the
differences between them. This is what we should be driven to, if we were
ever to succeed in collecting all the forms in any one class which have
lived throughout all time and space. Assuredly we shall never succeed in
making so perfect a collection: nevertheless, in certain classes, we are
tending toward this end; and Milne Edwards has lately insisted, in an able
paper, on the high importance of looking to types, whether or not we can
separate and define the groups to which such types belong.
Finally, we have seen that natural selection, which follows from the
struggle for existence, and which almost inevitably leads to extinction and
divergence of character in the descendants from any one parent-species,
explains that great and universal feature in the affinities of all organic
beings, namely, their subordination in group under group. We use the
element of descent in classing the individuals of both sexes and of all
ages under one species, although they may have but few characters in
common; we use descent in classing acknowledged varieties, however
different they may be from their parents; and I believe that this element
of descent is the hidden bond of connexion which naturalists have sought
under the term of the Natural System. On this idea of the natural system
being, in so far as it has been perfected, genealogical in its arrangement,
with the grades of difference expressed by the terms genera, families,
orders, etc., we can understand the rules which we are compelled to follow
in our classification. We can understand why we value certain resemblances
far more than others; why we use rudimentary and useless organs, or others
of trifling physiological importance; why, in finding the relations between
one group and another, we summarily reject analogical or adaptive
characters, and yet use these same characters within the limits of the same
group. We can clearly see how it is that all living and extinct forms can
be grouped together within a few great classes; and how the several members
of each class are connected together by the most complex and radiating
lines of affinities. We shall never, probably, disentangle the
inextricable web of the affinities between the members of any one class;
but when we have a distinct object in view, and do not look to some unknown
plan of creation, we may hope to make sure but slow progress.
Professor Haeckel in his "Generelle Morphologie" and in another works, has
recently brought his great knowledge and abilities to bear on what he calls
phylogeny, or the lines of descent of all organic beings. In drawing up
the several series he trusts chiefly to embryological characters, but
receives aid from homologous and rudimentary organs, as well as from the
successive periods at which the various forms of life are believed to have
first appeared in our geological formations. He has thus boldly made a
great beginning, and shows us how classification will in the future be
treated.
MORPHOLOGY.
We have seen that the members of the same class, independently of their
habits of life, resemble each other in the general plan of their
organisation. This resemblance is often expressed by the term "unity of
type;" or by saying that the several parts and organs in the different
species of the class are homologous. The whole subject is included under
the general term of Morphology. This is one of the most interesting
departments of natural history, and may almost be said to be its very soul.
What can be more curious than that the hand of a man, formed for grasping,
that of a mole for digging, the leg of the horse, the paddle of the
porpoise, and the wing of the bat, should all be constructed on the same
pattern, and should include similar bones, in the same relative positions?
How curious it is, to give a subordinate though striking instance, that the
hind feet of the kangaroo, which are so well fitted for bounding over the
open plains--those of the climbing, leaf-eating koala, equally well fitted
for grasping the branches of trees--those of the ground-dwelling, insect or
root-eating, bandicoots--and those of some other Australian marsupials--
should all be constructed on the same extraordinary type, namely with the
bones of the second and third digits extremely slender and enveloped within
the same skin, so that they appear like a single toe furnished with two
claws. Notwithstanding this similarity of pattern, it is obvious that the
hind feet of these several animals are used for as widely different
purposes as it is possible to conceive. The case is rendered all the more
striking by the American opossums, which follow nearly the same habits of
life as some of their Australian relatives, having feet constructed on the
ordinary plan. Professor Flower, from whom these statements are taken,
remarks in conclusion: "We may call this conformity to type, without
getting much nearer to an explanation of the phenomenon;" and he then adds
"but is it not powerfully suggestive of true relationship, of inheritance
from a common ancestor?"
Geoffroy St. Hilaire has strongly insisted on the high importance of
relative position or connexion in homologous parts; they may differ to
almost any extent in form and size, and yet remain connected together in
the same invariable order. We never find, for instance, the bones of the
arm and forearm, or of the thigh and leg, transposed. Hence the same names
can be given to the homologous bones in widely different animals. We see
the same great law in the construction of the mouths of insects: what can
be more different than the immensely long spiral proboscis of a
sphinx-moth, the curious folded one of a bee or bug, and the great jaws of
a beetle? Yet all these organs, serving for such widely different
purposes, are formed by infinitely numerous modifications of an upper lip,
mandibles, and two pairs of maxillae. The same law governs the
construction of the mouths and limbs of crustaceans. So it is with the
flowers of plants.
Nothing can be more hopeless than to attempt to explain this similarity of
pattern in members of the same class, by utility or by the doctrine of
final causes. The hopelessness of the attempt has been expressly admitted
by Owen in his most interesting work on the "Nature of Limbs." On the
ordinary view of the independent creation of each being, we can only say
that so it is; that it has pleased the Creator to construct all the animals
and plants in each great class on a uniform plan; but this is not a
scientific explanation.
The explanation is to a large extent simple, on the theory of the selection
of successive slight modifications, each being profitable in some way to
the modified form, but often affecting by correlation other parts of the
organisation. In changes of this nature, there will be little or no
tendency to alter the original pattern, or to transpose the parts. The
bones of a limb might be shortened and flattened to any extent, becoming at
the same time enveloped in thick membrane, so as to serve as a fin; or a
webbed hand might have all its bones, or certain bones, lengthened to any
extent, with the membrane connecting them increased, so as to serve as a
wing; yet all these modifications would not tend to alter the framework of
the bones or the relative connexion of the parts. If we suppose that an
early progenitor--the archetype, as it may be called--of all mammals, birds
and reptiles, had its limbs constructed on the existing general pattern,
for whatever purpose they served, we can at once perceive the plain
signification of the homologous construction of the limbs throughout the
class. So with the mouths of insects, we have only to suppose that their
common progenitor had an upper lip, mandibles, and two pairs of maxillae,
these parts being perhaps very simple in form; and then natural selection
will account for the infinite diversity in structure and function of the
mouths of insects. Nevertheless, it is conceivable that the general
pattern of an organ might become so much obscured as to be finally lost, by
the reduction and ultimately by the complete abortion of certain parts, by
the fusion of other parts, and by the doubling or multiplication of others,
variations which we know to be within the limits of possibility. In the
paddles of the gigantic extinct sea-lizards, and in the mouths of certain
suctorial crustaceans, the general pattern seems thus to have become
partially obscured.
There is another and equally curious branch of our subject; namely, serial
homologies, or the comparison of the different parts or organs in the same
individual, and not of the same parts or organs in different members of the
same class. Most physiologists believe that the bones of the skull are
homologous--that is, correspond in number and in relative connexion--with
the elemental parts of a certain number of vertebrae. The anterior and
posterior limbs in all the higher vertebrate classes are plainly
homologous. So it is with the wonderfully complex jaws and legs of
crustaceans. It is familiar to almost every one, that in a flower the
relative position of the sepals, petals, stamens, and pistils, as well as
their intimate structure, are intelligible on the view that they consist of
metamorphosed leaves, arranged in a spire. In monstrous plants, we often
get direct evidence of the possibility of one organ being transformed into
another; and we can actually see, during the early or embryonic stages of
development in flowers, as well as in crustaceans and many other animals,
that organs, which when mature become extremely different are at first
exactly alike.
How inexplicable are the cases of serial homologies on the ordinary view of
creation! Why should the brain be enclosed in a box composed of such
numerous and such extraordinarily shaped pieces of bone apparently
representing vertebrae? As Owen has remarked, the benefit derived from the
yielding of the separate pieces in the act of parturition by mammals, will
by no means explain the same construction in the skulls of birds and
reptiles. Why should similar bones have been created to form the wing and
the leg of a bat, used as they are for such totally different purposes,
namely flying and walking? Why should one crustacean, which has an
extremely complex mouth formed of many parts, consequently always have
fewer legs; or conversely, those with many legs have simpler mouths? Why
should the sepals, petals, stamens, and pistils, in each flower, though
fitted for such distinct purposes, be all constructed on the same pattern?
On the theory of natural selection, we can, to a certain extent, answer
these questions. We need not here consider how the bodies of some animals
first became divided into a series of segments, or how they became divided
into right and left sides, with corresponding organs, for such questions
are almost beyond investigation. It is, however, probable that some serial
structures are the result of cells multiplying by division, entailing the
multiplication of the parts developed from such cells. It must suffice for
our purpose to bear in mind that an indefinite repetition of the same part
or organ is the common characteristic, as Owen has remarked, of all low or
little specialised forms; therefore the unknown progenitor of the
Vertebrata probably possessed many vertebrae; the unknown progenitor of the
Articulata, many segments; and the unknown progenitor of flowering plants,
many leaves arranged in one or more spires. We have also formerly seen
that parts many times repeated are eminently liable to vary, not only in
number, but in form. Consequently such parts, being already present in
considerable numbers, and being highly variable, would naturally afford the
materials for adaptation to the most different purposes; yet they would
generally retain, through the force of inheritance, plain traces of their
original or fundamental resemblance. They would retain this resemblance
all the more, as the variations, which afforded the basis for their
subsequent modification through natural selection, would tend from the
first to be similar; the parts being at an early stage of growth alike, and
being subjected to nearly the same conditions. Such parts, whether more or
less modified, unless their common origin became wholly obscured, would be
serially homologous.
In the great class of molluscs, though the parts in distinct species can be
shown to be homologous, only a few serial homologies; such as the valves of
Chitons, can be indicated; that is, we are seldom enabled to say that one
part is homologous with another part in the same individual. And we can
understand this fact; for in molluscs, even in the lowest members of the
class, we do not find nearly so much indefinite repetition of any one part
as we find in the other great classes of the animal and vegetable kingdoms.
But morphology is a much more complex subject than it at first appears, as
has lately been well shown in a remarkable paper by Mr. E. Ray Lankester,
who has drawn an important distinction between certain classes of cases
which have all been equally ranked by naturalists as homologous. He
proposes to call the structures which resemble each other in distinct
animals, owing to their descent from a common progenitor with subsequent
modification, "homogenous"; and the resemblances which cannot thus be
accounted for, he proposes to call "homoplastic". For instance, he
believes that the hearts of birds and mammals are as a whole homogenous--
that is, have been derived from a common progenitor; but that the four
cavities of the heart in the two classes are homoplastic--that is, have
been independently developed. Mr. Lankester also adduces the close
resemblance of the parts on the right and left sides of the body, and in
the successive segments of the same individual animal; and here we have
parts commonly called homologous which bear no relation to the descent of
distinct species from a common progenitor. Homoplastic structures are the
same with those which I have classed, though in a very imperfect manner, as
analogous modifications or resemblances. Their formation may be attributed
in part to distinct organisms, or to distinct parts of the same organism,
having varied in an analogous manner; and in part to similar modifications,
having been preserved for the same general purpose or function, of which
many instances have been given.
Naturalists frequently speak of the skull as formed of metamorphosed
vertebrae; the jaws of crabs as metamorphosed legs; the stamens and pistils
in flowers as metamorphosed leaves; but it would in most cases be more
correct, as Professor Huxley has remarked, to speak of both skull and
vertebrae, jaws and legs, etc., as having been metamorphosed, not one from
the other, as they now exist, but from some common and simpler element.
Most naturalists, however, use such language only in a metaphorical sense:
they are far from meaning that during a long course of descent, primordial
organs of any kind--vertebrae in the one case and legs in the other--have
actually been converted into skulls or jaws. Yet so strong is the
appearance of this having occurred that naturalists can hardly avoid
employing language having this plain signification. According to the views
here maintained, such language may be used literally; and the wonderful
fact of the jaws, for instance, of a crab retaining numerous characters,
which they probably would have retained through inheritance, if they had
really been metamorphosed from true though extremely simple legs, is in
part explained.
DEVELOPMENT AND EMBRYOLOGY.
This is one of the most important subjects in the whole round of natural
history. The metamorphoses of insects, with which every one is familiar,
are generally effected abruptly by a few stages; but the transformations
are in reality numerous and gradual, though concealed. A certain
ephemerous insect (Chloeon) during its development, moults, as shown by Sir
J. Lubbock, above twenty times, and each time undergoes a certain amount of
change; and in this case we see the act of metamorphosis performed in a
primary and gradual manner. Many insects, and especially certain
crustaceans, show us what wonderful changes of structure can be effected
during development. Such changes, however, reach their acme in the so-
called alternate generations of some of the lower animals. It is, for
instance, an astonishing fact that a delicate branching coralline, studded
with polypi, and attached to a submarine rock, should produce, first by
budding and then by transverse division, a host of huge floating jelly-
fishes; and that these should produce eggs, from which are hatched swimming
animalcules, which attach themselves to rocks and become developed into
branching corallines; and so on in an endless cycle. The belief in the
essential identity of the process of alternate generation and of ordinary
metamorphosis has been greatly strengthened by Wagner's discovery of the
larva or maggot of a fly, namely the Cecidomyia, producing asexually other
larvae, and these others, which finally are developed into mature males and
females, propagating their kind in the ordinary manner by eggs.
It may be worth notice that when Wagner's remarkable discovery was first
announced, I was asked how was it possible to account for the larvae of
this fly having acquired the power of a sexual reproduction. As long as
the case remained unique no answer could be given. But already Grimm has
shown that another fly, a Chironomus, reproduces itself in nearly the same
manner, and he believes that this occurs frequently in the order. It is
the pupa, and not the larva, of the Chironomus which has this power; and
Grimm further shows that this case, to a certain extent, "unites that of
the Cecidomyia with the parthenogenesis of the Coccidae;" the term
parthenogenesis implying that the mature females of the Coccidae are
capable of producing fertile eggs without the concourse of the male.
Certain animals belonging to several classes are now known to have the
power of ordinary reproduction at an unusually early age; and we have only
to accelerate parthenogenetic reproduction by gradual steps to an earlier
and earlier age--Chironomus showing us an almost exactly intermediate
stage, viz., that of the pupa--and we can perhaps account for the
marvellous case of the Cecidomyia.
It has already been stated that various parts in the same individual, which
are exactly alike during an early embryonic period, become widely different
and serve for widely different purposes in the adult state. So again it
has been shown that generally the embryos of the most distinct species
belonging to the same class are closely similar, but become, when fully
developed, widely dissimilar. A better proof of this latter fact cannot be
given than the statement by Von Baer that "the embryos of mammalia, of
birds, lizards and snakes, probably also of chelonia, are in the earliest
states exceedingly like one another, both as a whole and in the mode of
development of their parts; so much so, in fact, that we can often
distinguish the embryos only by their size. In my possession are two
little embryos in spirit, whose names I have omitted to attach, and at
present I am quite unable to say to what class they belong. They may be
lizards or small birds, or very young mammalia, so complete is the
similarity in the mode of formation of the head and trunk in these animals.
The extremities, however, are still absent in these embryos. But even if
they had existed in the earliest stage of their development we should learn
nothing, for the feet of lizards and mammals, the wings and feet of birds,
no less than the hands and feet of man, all arise from the same fundamental
form." The larvae of most crustaceans, at corresponding stages of
development, closely resemble each other, however different the adults may
become; and so it is with very many other animals. A trace of the law of
embryonic resemblance occasionally lasts till a rather late age: thus
birds of the same genus, and of allied genera, often resemble each other in
their immature plumage; as we see in the spotted feathers in the young of
the thrush group. In the cat tribe, most of the species when adult are
striped or spotted in lines; and stripes or spots can be plainly
distinguished in the whelp of the lion and the puma. We occasionally,
though rarely, see something of the same kind in plants; thus the first
leaves of the ulex or furze, and the first leaves of the phyllodineous
acacias, are pinnate or divided like the ordinary leaves of the
leguminosae.
The points of structure, in which the embryos of widely different animals
within the same class resemble each other, often have no direct relation to
their conditions of existence. We cannot, for instance, suppose that in
the embryos of the vertebrata the peculiar loop-like courses of the
arteries near the branchial slits are related to similar conditions--in the
young mammal which is nourished in the womb of its mother, in the egg of
the bird which is hatched in a nest, and in the spawn of a frog under
water. We have no more reason to believe in such a relation than we have
to believe that the similar bones in the hand of a man, wing of a bat, and
fin of a porpoise, are related to similar conditions of life. No one
supposes that the stripes on the whelp of a lion, or the spots on the young
blackbird, are of any use to these animals.
The case, however, is different when an animal, during any part of its
embryonic career, is active, and has to provide for itself. The period of
activity may come on earlier or later in life; but whenever it comes on,
the adaptation of the larva to its conditions of life is just as perfect
and as beautiful as in the adult animal. In how important a manner this
has acted, has recently been well shown by Sir J. Lubbock in his remarks on
the close similarity of the larvae of some insects belonging to very
different orders, and on the dissimilarity of the larvae of other insects
within the same order, according to their habits of life. Owing to such
adaptations the similarity of the larvae of allied animals is sometimes
greatly obscured; especially when there is a division of labour during the
different stages of development, as when the same larva has during one
stage to search for food, and during another stage has to search for a
place of attachment. Cases can even be given of the larvae of allied
species, or groups of species, differing more from each other than do the
adults. In most cases, however, the larvae, though active, still obey,
more or less closely, the law of common embryonic resemblance. Cirripedes
afford a good instance of this: even the illustrious Cuvier did not
perceive that a barnacle was a crustacean: but a glance at the larva shows
this in an unmistakable manner. So again the two main divisions of
cirripedes, the pedunculated and sessile, though differing widely in
external appearance, have larvae in all their stages barely
distinguishable.
The embryo in the course of development generally rises in organisation. I
use this expression, though I am aware that it is hardly possible to define
clearly what is meant by organisation being higher or lower. But no one
probably will dispute that the butterfly is higher than the caterpillar.
In some cases, however, the mature animal must be considered as lower in
the scale than the larva, as with certain parasitic crustaceans. To refer
once again to cirripedes: the larvae in the first stage have three pairs
of locomotive organs, a simple single eye, and a probosciformed mouth, with
which they feed largely, for they increase much in size. In the second
stage, answering to the chrysalis stage of butterflies, they have six pairs
of beautifully constructed natatory legs, a pair of magnificent compound
eyes, and extremely complex antennae; but they have a closed and imperfect
mouth, and cannot feed: their function at this stage is, to search out by
their well-developed organs of sense, and to reach by their active powers
of swimming, a proper place on which to become attached and to undergo
their final metamorphosis. When this is completed they are fixed for life:
their legs are now converted into prehensile organs; they again obtain a
well-constructed mouth; but they have no antennae, and their two eyes are
now reconverted into a minute, single, simple eye-spot. In this last and
complete state, cirripedes may be considered as either more highly or more
lowly organised than they were in the larval condition. But in some genera
the larvae become developed into hermaphrodites having the ordinary
structure, or into what I have called complemental males; and in the latter
the development has assuredly been retrograde; for the male is a mere sack,
which lives for a short time and is destitute of mouth, stomach, and every
other organ of importance, excepting those for reproduction.
We are so much accustomed to see a difference in structure between the
embryo and the adult, that we are tempted to look at this difference as in
some necessary manner contingent on growth. But there is no reason why,
for instance, the wing of a bat, or the fin of a porpoise, should not have
been sketched out with all their parts in proper proportion, as soon as any
part became visible. In some whole groups of animals and in certain
members of other groups this is the case, and the embryo does not at any
period differ widely from the adult: thus Owen has remarked in regard to
cuttle-fish, "there is no metamorphosis; the cephalopodic character is
manifested long before the parts of the embryo are completed." Land-shells
and fresh-water crustaceans are born having their proper forms, while the
marine members of the same two great classes pass through considerable and
often great changes during their development. Spiders, again, barely
undergo any metamorphosis. The larvae of most insects pass through a
worm-like stage, whether they are active and adapted to diversified habits,
or are inactive from being placed in the midst of proper nutriment, or from
being fed by their parents; but in some few cases, as in that of Aphis, if
we look to the admirable drawings of the development of this insect, by
Professor Huxley, we see hardly any trace of the vermiform stage.
Sometimes it is only the earlier developmental stages which fail. Thus,
Fritz Muller has made the remarkable discovery that certain shrimp-like
crustaceans (allied to Penoeus) first appear under the simple nauplius-
form, and after passing through two or more zoea-stages, and then through
the mysis-stage, finally acquire their mature structure: now in the whole
great malacostracan order, to which these crustaceans belong, no other
member is as yet known to be first developed under the nauplius-form,
though many appear as zoeas; nevertheless Muller assigns reasons for his
belief, that if there had been no suppression of development, all these
crustaceans would have appeared as nauplii.
How, then, can we explain these several facts in embryology--namely, the
very general, though not universal, difference in structure between the
embryo and the adult; the various parts in the same individual embryo,
which ultimately become very unlike, and serve for diverse purposes, being
at an early period of growth alike; the common, but not invariable,
resemblance between the embryos or larvae of the most distinct species in
the same class; the embryo often retaining, while within the egg or womb,
structures which are of no service to it, either at that or at a later
period of life; on the other hand, larvae which have to provide for their
own wants, being perfectly adapted to the surrounding conditions; and
lastly, the fact of certain larvae standing higher in the scale of
organisation than the mature animal into which they are developed? I
believe that all these facts can be explained as follows.
It is commonly assumed, perhaps from monstrosities affecting the embryo at
a very early period, that slight variations or individual differences
necessarily appear at an equally early period. We have little evidence on
this head, but what we have certainly points the other way; for it is
notorious that breeders of cattle, horses and various fancy animals, cannot
positively tell, until some time after birth, what will be the merits and
demerits of their young animals. We see this plainly in our own children;
we cannot tell whether a child will be tall or short, or what its precise
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