|
Charles Darwin: The origin of life
For those without javascript use this menu
CHAPTER II.
VARIATION UNDER NATURE.
Variability -- Individual differences -- Doubtful species -- Wide ranging,
much diffused, and common species, vary most -- Species of the larger
genera in each country vary more frequently than the species of the smaller
genera -- Many of the species of the larger genera resemble varieties in
being very closely, but unequally, related to each other, and in having
restricted ranges.
Before applying the principles arrived at in the last chapter to organic
beings in a state of nature, we must briefly discuss whether these latter
are subject to any variation. To treat this subject properly, a long
catalogue of dry facts ought to be given; but these I shall reserve for a
future work. Nor shall I here discuss the various definitions which have
been given of the term species. No one definition has satisfied all
naturalists; yet every naturalist knows vaguely what he means when he
speaks of a species. Generally the term includes the unknown element of a
distinct act of creation. The term "variety" is almost equally difficult
to define; but here community of descent is almost universally implied,
though it can rarely be proved. We have also what are called
monstrosities; but they graduate into varieties. By a monstrosity I
presume is meant some considerable deviation of structure, generally
injurious, or not useful to the species. Some authors use the term
"variation" in a technical sense, as implying a modification directly due
to the physical conditions of life; and "variations" in this sense are
supposed not to be inherited; but who can say that the dwarfed condition of
shells in the brackish waters of the Baltic, or dwarfed plants on Alpine
summits, or the thicker fur of an animal from far northwards, would not in
some cases be inherited for at least a few generations? And in this case I
presume that the form would be called a variety.
It may be doubted whether sudden and considerable deviations of structure,
such as we occasionally see in our domestic productions, more especially
with plants, are ever permanently propagated in a state of nature. Almost
every part of every organic being is so beautifully related to its complex
conditions of life that it seems as improbable that any part should have
been suddenly produced perfect, as that a complex machine should have been
invented by man in a perfect state. Under domestication monstrosities
sometimes occur which resemble normal structures in widely different
animals. Thus pigs have occasionally been born with a sort of proboscis,
and if any wild species of the same genus had naturally possessed a
proboscis, it might have been argued that this had appeared as a
monstrosity; but I have as yet failed to find, after diligent search, cases
of monstrosities resembling normal structures in nearly allied forms, and
these alone bear on the question. If monstrous forms of this kind ever do
appear in a state of nature and are capable of reproduction (which is not
always the case), as they occur rarely and singly, their preservation would
depend on unusually favourable circumstances. They would, also, during the
first and succeeding generations cross with the ordinary form, and thus
their abnormal character would almost inevitably be lost. But I shall have
to return in a future chapter to the preservation and perpetuation of
single or occasional variations.
INDIVIDUAL DIFFERENCES.
The many slight differences which appear in the offspring from the same
parents, or which it may be presumed have thus arisen, from being observed
in the individuals of the same species inhabiting the same confined
locality, may be called individual differences. No one supposes that all
the individuals of the same species are cast in the same actual mould.
These individual differences are of the highest importance for us, for they
are often inherited, as must be familiar to every one; and they thus afford
materials for natural selection to act on and accumulate, in the same
manner as man accumulates in any given direction individual differences in
his domesticated productions. These individual differences generally
affect what naturalists consider unimportant parts; but I could show, by a
long catalogue of facts, that parts which must be called important, whether
viewed under a physiological or classificatory point of view, sometimes
vary in the individuals of the same species. I am convinced that the most
experienced naturalist would be surprised at the number of the cases of
variability, even in important parts of structure, which he could collect
on good authority, as I have collected, during a course of years. It
should be remembered that systematists are far from being pleased at
finding variability in important characters, and that there are not many
men who will laboriously examine internal and important organs, and compare
them in many specimens of the same species. It would never have been
expected that the branching of the main nerves close to the great central
ganglion of an insect would have been variable in the same species; it
might have been thought that changes of this nature could have been
effected only by slow degrees; yet Sir J. Lubbock has shown a degree of
variability in these main nerves in Coccus, which may almost be compared to
the irregular branching of the stem of a tree. This philosophical
naturalist, I may add, has also shown that the muscles in the larvae of
certain insects are far from uniform. Authors sometimes argue in a circle
when they state that important organs never vary; for these same authors
practically rank those parts as important (as some few naturalists have
honestly confessed) which do not vary; and, under this point of view, no
instance will ever be found of an important part varying; but under any
other point of view many instances assuredly can be given.
There is one point connected with individual differences which is extremely
perplexing: I refer to those genera which have been called "protean" or
"polymorphic," in which species present an inordinate amount of variation.
With respect to many of these forms, hardly two naturalists agree whether
to rank them as species or as varieties. We may instance Rubus, Rosa, and
Hieracium among plants, several genera of insects, and of Brachiopod
shells. In most polymorphic genera some of the species have fixed and
definite characters. Genera which are polymorphic in one country seem to
be, with a few exceptions, polymorphic in other countries, and likewise,
judging from Brachiopod shells, at former periods of time. These facts are
very perplexing, for they seem to show that this kind of variability is
independent of the conditions of life. I am inclined to suspect that we
see, at least in some of these polymorphic genera, variations which are of
no service or disservice to the species, and which consequently have not
been seized on and rendered definite by natural selection, as hereafter to
be explained.
Individuals of the same species often present, as is known to every one,
great differences of structure, independently of variation, as in the two
sexes of various animals, in the two or three castes of sterile females or
workers among insects, and in the immature and larval states of many of the
lower animals. There are, also, cases of dimorphism and trimorphism, both
with animals and plants. Thus, Mr. Wallace, who has lately called
attention to the subject, has shown that the females of certain species of
butterflies, in the Malayan Archipelago, regularly appear under two or even
three conspicuously distinct forms, not connected by intermediate
varieties. Fritz Muller has described analogous but more extraordinary
cases with the males of certain Brazilian Crustaceans: thus, the male of a
Tanais regularly occurs under two distinct forms; one of these has strong
and differently shaped pincers, and the other has antennae much more
abundantly furnished with smelling-hairs. Although in most of these cases,
the two or three forms, both with animals and plants, are not now connected
by intermediate gradations, it is possible that they were once thus
connected. Mr. Wallace, for instance, describes a certain butterfly which
presents in the same island a great range of varieties connected by
intermediate links, and the extreme links of the chain closely resemble the
two forms of an allied dimorphic species inhabiting another part of the
Malay Archipelago. Thus also with ants, the several worker-castes are
generally quite distinct; but in some cases, as we shall hereafter see, the
castes are connected together by finely graduated varieties. So it is, as
I have myself observed, with some dimorphic plants. It certainly at first
appears a highly remarkable fact that the same female butterfly should have
the power of producing at the same time three distinct female forms and a
male; and that an hermaphrodite plant should produce from the same seed-
capsule three distinct hermaphrodite forms, bearing three different kinds
of females and three or even six different kinds of males. Nevertheless
these cases are only exaggerations of the common fact that the female
produces offspring of two sexes which sometimes differ from each other in a
wonderful manner.
DOUBTFUL SPECIES.
The forms which possess in some considerable degree the character of
species, but which are so closely similar to other forms, or are so closely
linked to them by intermediate gradations, that naturalists do not like to
rank them as distinct species, are in several respects the most important
for us. We have every reason to believe that many of these doubtful and
closely allied forms have permanently retained their characters for a long
time; for as long, as far as we know, as have good and true species.
Practically, when a naturalist can unite by means of intermediate links any
two forms, he treats the one as a variety of the other, ranking the most
common, but sometimes the one first described as the species, and the other
as the variety. But cases of great difficulty, which I will not here
enumerate, sometimes arise in deciding whether or not to rank one form as a
variety of another, even when they are closely connected by intermediate
links; nor will the commonly assumed hybrid nature of the intermediate
forms always remove the difficulty. In very many cases, however, one form
is ranked as a variety of another, not because the intermediate links have
actually been found, but because analogy leads the observer to suppose
either that they do now somewhere exist, or may formerly have existed; and
here a wide door for the entry of doubt and conjecture is opened.
Hence, in determining whether a form should be ranked as a species or a
variety, the opinion of naturalists having sound judgment and wide
experience seems the only guide to follow. We must, however, in many
cases, decide by a majority of naturalists, for few well-marked and
well-known varieties can be named which have not been ranked as species by
at least some competent judges.
That varieties of this doubtful nature are far from uncommon cannot be
disputed. Compare the several floras of Great Britain, of France, or of
the United States, drawn up by different botanists, and see what a
surprising number of forms have been ranked by one botanist as good
species, and by another as mere varieties. Mr. H.C. Watson, to whom I lie
under deep obligation for assistance of all kinds, has marked for me 182
British plants, which are generally considered as varieties, but which have
all been ranked by botanists as species; and in making this list he has
omitted many trifling varieties, but which nevertheless have been ranked by
some botanists as species, and he has entirely omitted several highly
polymorphic genera. Under genera, including the most polymorphic forms,
Mr. Babington gives 251 species, whereas Mr. Bentham gives only 112--a
difference of 139 doubtful forms! Among animals which unite for each
birth, and which are highly locomotive, doubtful forms, ranked by one
zoologist as a species and by another as a variety, can rarely be found
within the same country, but are common in separated areas. How many of
the birds and insects in North America and Europe, which differ very
slightly from each other, have been ranked by one eminent naturalist as
undoubted species, and by another as varieties, or, as they are often
called, geographical races! Mr. Wallace, in several valuable papers on the
various animals, especially on the Lepidoptera, inhabiting the islands of
the great Malayan Archipelago, shows that they may be classed under four
heads, namely, as variable forms, as local forms, as geographical races or
sub-species, and as true representative species. The first or variable
forms vary much within the limits of the same island. The local forms are
moderately constant and distinct in each separate island; but when all from
the several islands are compared together, the differences are seen to be
so slight and graduated that it is impossible to define or describe them,
though at the same time the extreme forms are sufficiently distinct. The
geographical races or sub-species are local forms completely fixed and
isolated; but as they do not differ from each other by strongly marked and
important characters, "There is no possible test but individual opinion to
determine which of them shall be considered as species and which as
varieties." Lastly, representative species fill the same place in the
natural economy of each island as do the local forms and sub-species; but
as they are distinguished from each other by a greater amount of difference
than that between the local forms and sub-species, they are almost
universally ranked by naturalists as true species. Nevertheless, no
certain criterion can possibly be given by which variable forms, local
forms, sub species and representative species can be recognised.
Many years ago, when comparing, and seeing others compare, the birds from
the closely neighbouring islands of the Galapagos Archipelago, one with
another, and with those from the American mainland, I was much struck how
entirely vague and arbitrary is the distinction between species and
varieties. On the islets of the little Madeira group there are many
insects which are characterized as varieties in Mr. Wollaston's admirable
work, but which would certainly be ranked as distinct species by many
entomologists. Even Ireland has a few animals, now generally regarded as
varieties, but which have been ranked as species by some zoologists.
Several experienced ornithologists consider our British red grouse as only
a strongly marked race of a Norwegian species, whereas the greater number
rank it as an undoubted species peculiar to Great Britain. A wide distance
between the homes of two doubtful forms leads many naturalists to rank them
as distinct species; but what distance, it has been well asked, will
suffice if that between America and Europe is ample, will that between
Europe and the Azores, or Madeira, or the Canaries, or between the several
islets of these small archipelagos, be sufficient?
Mr. B.D. Walsh, a distinguished entomologist of the United States, has
described what he calls Phytophagic varieties and Phytophagic species.
Most vegetable-feeding insects live on one kind of plant or on one group of
plants; some feed indiscriminately on many kinds, but do not in consequence
vary. In several cases, however, insects found living on different plants,
have been observed by Mr. Walsh to present in their larval or mature state,
or in both states, slight, though constant differences in colour, size, or
in the nature of their secretions. In some instances the males alone, in
other instances, both males and females, have been observed thus to differ
in a slight degree. When the differences are rather more strongly marked,
and when both sexes and all ages are affected, the forms are ranked by all
entomologists as good species. But no observer can determine for another,
even if he can do so for himself, which of these Phytophagic forms ought to
be called species and which varieties. Mr. Walsh ranks the forms which it
may be supposed would freely intercross, as varieties; and those which
appear to have lost this power, as species. As the differences depend on
the insects having long fed on distinct plants, it cannot be expected that
intermediate links connecting the several forms should now be found. The
naturalist thus loses his best guide in determining whether to rank
doubtful forms as varieties or species. This likewise necessarily occurs
with closely allied organisms, which inhabit distinct continents or
islands. When, on the other hand, an animal or plant ranges over the same
continent, or inhabits many islands in the same archipelago, and presents
different forms in the different areas, there is always a good chance that
intermediate forms will be discovered which will link together the extreme
states; and these are then degraded to the rank of varieties.
Some few naturalists maintain that animals never present varieties; but
then these same naturalists rank the slightest difference as of specific
value; and when the same identical form is met with in two distant
countries, or in two geological formations, they believe that two distinct
species are hidden under the same dress. The term species thus comes to be
a mere useless abstraction, implying and assuming a separate act of
creation. It is certain that many forms, considered by highly competent
judges to be varieties, resemble species so completely in character that
they have been thus ranked by other highly competent judges. But to
discuss whether they ought to be called species or varieties, before any
definition of these terms has been generally accepted, is vainly to beat
the air.
Many of the cases of strongly marked varieties or doubtful species well
deserve consideration; for several interesting lines of argument, from
geographical distribution, analogical variation, hybridism, etc., have been
brought to bear in the attempt to determine their rank; but space does not
here permit me to discuss them. Close investigation, in many cases, will
no doubt bring naturalists to agree how to rank doubtful forms. Yet it
must be confessed that it is in the best known countries that we find the
greatest number of them. I have been struck with the fact that if any
animal or plant in a state of nature be highly useful to man, or from any
cause closely attracts his attention, varieties of it will almost
universally be found recorded. These varieties, moreover, will often be
ranked by some authors as species. Look at the common oak, how closely it
has been studied; yet a German author makes more than a dozen species out
of forms, which are almost universally considered by other botanists to be
varieties; and in this country the highest botanical authorities and
practical men can be quoted to show that the sessile and pedunculated oaks
are either good and distinct species or mere varieties.
I may here allude to a remarkable memoir lately published by A. de
Candolle, on the oaks of the whole world. No one ever had more ample
materials for the discrimination of the species, or could have worked on
them with more zeal and sagacity. He first gives in detail all the many
points of structure which vary in the several species, and estimates
numerically the relative frequency of the variations. He specifies above a
dozen characters which may be found varying even on the same branch,
sometimes according to age or development, sometimes without any assignable
reason. Such characters are not of course of specific value, but they are,
as Asa Gray has remarked in commenting on this memoir, such as generally
enter into specific definitions. De Candolle then goes on to say that he
gives the rank of species to the forms that differ by characters never
varying on the same tree, and never found connected by intermediate states.
After this discussion, the result of so much labour, he emphatically
remarks: "They are mistaken, who repeat that the greater part of our
species are clearly limited, and that the doubtful species are in a feeble
minority. This seemed to be true, so long as a genus was imperfectly
known, and its species were founded upon a few specimens, that is to say,
were provisional. Just as we come to know them better, intermediate forms
flow in, and doubts as to specific limits augment." He also adds that it
is the best known species which present the greatest number of spontaneous
varieties and sub-varieties. Thus Quercus robur has twenty-eight
varieties, all of which, excepting six, are clustered round three sub-
species, namely Q. pedunculata, sessiliflora and pubescens. The forms
which connect these three sub-species are comparatively rare; and, as Asa
Gray again remarks, if these connecting forms which are now rare were to
become totally extinct the three sub-species would hold exactly the same
relation to each other as do the four or five provisionally admitted
species which closely surround the typical Quercus robur. Finally, De
Candolle admits that out of the 300 species, which will be enumerated in
his Prodromus as belonging to the oak family, at least two-thirds are
provisional species, that is, are not known strictly to fulfil the
definition above given of a true species. It should be added that De
Candolle no longer believes that species are immutable creations, but
concludes that the derivative theory is the most natural one, "and the most
accordant with the known facts in palaeontology, geographical botany and
zoology, of anatomical structure and classification."
When a young naturalist commences the study of a group of organisms quite
unknown to him he is at first much perplexed in determining what
differences to consider as specific and what as varietal; for he knows
nothing of the amount and kind of variation to which the group is subject;
and this shows, at least, how very generally there is some variation. But
if he confine his attention to one class within one country he will soon
make up his mind how to rank most of the doubtful forms. His general
tendency will be to make many species, for he will become impressed, just
like the pigeon or poultry fancier before alluded to, with the amount of
difference in the forms which he is continually studying; and he has little
general knowledge of analogical variation in other groups and in other
countries by which to correct his first impressions. As he extends the
range of his observations he will meet with more cases of difficulty; for
he will encounter a greater number of closely-allied forms. But if his
observations be widely extended he will in the end generally be able to
make up his own mind; but he will succeed in this at the expense of
admitting much variation, and the truth of this admission will often be
disputed by other naturalists. When he comes to study allied forms brought
from countries not now continuous, in which case he cannot hope to find
intermediate links, he will be compelled to trust almost entirely to
analogy, and his difficulties will rise to a climax.
Certainly no clear line of demarcation has as yet been drawn between
species and sub-species--that is, the forms which in the opinion of some
naturalists come very near to, but do not quite arrive at, the rank of
species; or, again, between sub-species and well-marked varieties, or
between lesser varieties and individual differences. These differences
blend into each other by an insensible series; and a series impresses the
mind with the idea of an actual passage.
Hence I look at individual differences, though of small interest to the
systematist, as of the highest importance for us, as being the first step
towards such slight varieties as are barely thought worth recording in
works on natural history. And I look at varieties which are in any degree
more distinct and permanent, as steps toward more strongly marked and
permanent varieties; and at the latter, as leading to sub-species, and then
to species. The passage from one stage of difference to another may, in
many cases, be the simple result of the nature of the organism and of the
different physical conditions to which it has long been exposed; but with
respect to the more important and adaptive characters, the passage from one
stage of difference to another may be safely attributed to the cumulative
action of natural selection, hereafter to be explained, and to the effects
of the increased use or disuse of parts. A well-marked variety may
therefore be called an incipient species; but whether this belief is
justifiable must be judged by the weight of the various facts and
considerations to be given throughout this work.
It need not be supposed that all varieties or incipient species attain the
rank of species. They may become extinct, or they may endure as varieties
for very long periods, as has been shown to be the case by Mr. Wollaston
with the varieties of certain fossil land-shells in Madeira, and with
plants by Gaston de Saporta. If a variety were to flourish so as to exceed
in numbers the parent species, it would then rank as the species, and the
species as the variety; or it might come to supplant and exterminate the
parent species; or both might co-exist, and both rank as independent
species. But we shall hereafter return to this subject.
From these remarks it will be seen that I look at the term species as one
arbitrarily given, for the sake of convenience, to a set of individuals
closely resembling each other, and that it does not essentially differ from
the term variety, which is given to less distinct and more fluctuating
forms. The term variety, again, in comparison with mere individual
differences, is also applied arbitrarily, for convenience sake.
WIDE-RANGING, MUCH DIFFUSED, AND COMMON SPECIES VARY MOST.
Guided by theoretical considerations, I thought that some interesting
results might be obtained in regard to the nature and relations of the
species which vary most, by tabulating all the varieties in several
well-worked floras. At first this seemed a simple task; but Mr. H.C.
Watson, to whom I am much indebted for valuable advice and assistance on
this subject, soon convinced me that there were many difficulties, as did
subsequently Dr. Hooker, even in stronger terms. I shall reserve for a
future work the discussion of these difficulties, and the tables of the
proportional numbers of the varying species. Dr. Hooker permits me to add
that after having carefully read my manuscript, and examined the tables, he
thinks that the following statements are fairly well established. The
whole subject, however, treated as it necessarily here is with much
brevity, is rather perplexing, and allusions cannot be avoided to the
"struggle for existence," "divergence of character," and other questions,
hereafter to be discussed.
Alphonse de Candolle and others have shown that plants which have very wide
ranges generally present varieties; and this might have been expected, as
they are exposed to diverse physical conditions, and as they come into
competition (which, as we shall hereafter see, is a far more important
circumstance) with different sets of organic beings. But my tables further
show that, in any limited country, the species which are the most common,
that is abound most in individuals, and the species which are most widely
diffused within their own country (and this is a different consideration
from wide range, and to a certain extent from commonness), oftenest give
rise to varieties sufficiently well-marked to have been recorded in
botanical works. Hence it is the most flourishing, or, as they may be
called, the dominant species--those which range widely, are the most
diffused in their own country, and are the most numerous in
individuals--which oftenest produce well-marked varieties, or, as I
consider them, incipient species. And this, perhaps, might have been
anticipated; for, as varieties, in order to become in any degree permanent,
necessarily have to struggle with the other inhabitants of the country, the
species which are already dominant will be the most likely to yield
offspring, which, though in some slight degree modified, still inherit
those advantages that enabled their parents to become dominant over their
compatriots. In these remarks on predominence, it should be understood
that reference is made only to the forms which come into competition with
each other, and more especially to the members of the same genus or class
having nearly similar habits of life. With respect to the number of
individuals or commonness of species, the comparison of course relates only
to the members of the same group. One of the higher plants may be said to
be dominant if it be more numerous in individuals and more widely diffused
than the other plants of the same country, which live under nearly the same
conditions. A plant of this kind is not the less dominant because some
conferva inhabiting the water or some parasitic fungus is infinitely more
numerous in individuals, and more widely diffused. But if the conferva or
parasitic fungus exceeds its allies in the above respects, it will then be
dominant within its own class.
SPECIES OF THE LARGER GENERA IN EACH COUNTRY VARY MORE FREQUENTLY THAN THE
SPECIES OF THE SMALLER GENERA.
If the plants inhabiting a country as described in any Flora, be divided
into two equal masses, all those in the larger genera (i.e., those
including many species) being placed on one side, and all those in the
smaller genera on the other side, the former will be found to include a
somewhat larger number of the very common and much diffused or dominant
species. This might have been anticipated, for the mere fact of many
species of the same genus inhabiting any country, shows that there is
something in the organic or inorganic conditions of that country favourable
to the genus; and, consequently, we might have expected to have found in
the larger genera, or those including many species, a larger proportional
number of dominant species. But so many causes tend to obscure this
result, that I am surprised that my tables show even a small majority on
the side of the larger genera. I will here allude to only two causes of
obscurity. Fresh water and salt-loving plants generally have very wide
ranges and are much diffused, but this seems to be connected with the
nature of the stations inhabited by them, and has little or no relation to
the size of the genera to which the species belong. Again, plants low in
the scale of organisation are generally much more widely diffused than
plants higher in the scale; and here again there is no close relation to
the size of the genera. The cause of lowly-organised plants ranging widely
will be discussed in our chapter on Geographical Distribution.
From looking at species as only strongly marked and well-defined varieties,
I was led to anticipate that the species of the larger genera in each
country would oftener present varieties, than the species of the smaller
genera; for wherever many closely related species (i.e., species of the
same genus) have been formed, many varieties or incipient species ought, as
a general rule, to be now forming. Where many large trees grow, we expect
to find saplings. Where many species of a genus have been formed through
variation, circumstances have been favourable for variation; and hence we
might expect that the circumstances would generally still be favourable to
variation. On the other hand, if we look at each species as a special act
of creation, there is no apparent reason why more varieties should occur in
a group having many species, than in one having few.
To test the truth of this anticipation I have arranged the plants of twelve
countries, and the coleopterous insects of two districts, into two nearly
equal masses, the species of the larger genera on one side, and those of
the smaller genera on the other side, and it has invariably proved to be
the case that a larger proportion of the species on the side of the larger
genera presented varieties, than on the side of the smaller genera.
Moreover, the species of the large genera which present any varieties,
invariably present a larger average number of varieties than do the species
of the small genera. Both these results follow when another division is
made, and when all the least genera, with from only one to four species,
are altogether excluded from the tables. These facts are of plain
signification on the view that species are only strongly marked and
permanent varieties; for wherever many species of the same genus have been
formed, or where, if we may use the expression, the manufactory of species
has been active, we ought generally to find the manufactory still in
action, more especially as we have every reason to believe the process of
manufacturing new species to be a slow one. And this certainly holds true
if varieties be looked at as incipient species; for my tables clearly show,
as a general rule, that, wherever many species of a genus have been formed,
the species of that genus present a number of varieties, that is, of
incipient species, beyond the average. It is not that all large genera are
now varying much, and are thus increasing in the number of their species,
or that no small genera are now varying and increasing; for if this had
been so, it would have been fatal to my theory; inasmuch as geology plainly
tells us that small genera have in the lapse of time often increased
greatly in size; and that large genera have often come to their maxima,
declined, and disappeared. All that we want to show is, that where many
species of a genus have been formed, on an average many are still forming;
and this certainly holds good.
MANY OF THE SPECIES INCLUDED WITHIN THE LARGER GENERA RESEMBLE VARIETIES IN
BEING VERY CLOSELY, BUT UNEQUALLY, RELATED TO EACH OTHER, AND IN HAVING
RESTRICTED RANGES.
There are other relations between the species of large genera and their
recorded varieties which deserve notice. We have seen that there is no
infallible criterion by which to distinguish species and well-marked
varieties; and when intermediate links have not been found between doubtful
forms, naturalists are compelled to come to a determination by the amount
of difference between them, judging by analogy whether or not the amount
suffices to raise one or both to the rank of species. Hence the amount of
difference is one very important criterion in settling whether two forms
should be ranked as species or varieties. Now Fries has remarked in regard
to plants, and Westwood in regard to insects, that in large genera the
amount of difference between the species is often exceedingly small. I
have endeavoured to test this numerically by averages, and, as far as my
imperfect results go, they confirm the view. I have also consulted some
sagacious and experienced observers, and, after deliberation, they concur
in this view. In this respect, therefore, the species of the larger genera
resemble varieties, more than do the species of the smaller genera. Or the
case may be put in another way, and it may be said, that in the larger
genera, in which a number of varieties or incipient species greater than
the average are now manufacturing, many of the species already manufactured
still to a certain extent resemble varieties, for they differ from each
other by a less than the usual amount of difference.
Moreover, the species of the larger genera are related to each other, in
the same manner as the varieties of any one species are related to each
other. No naturalist pretends that all the species of a genus are equally
distinct from each other; they may generally be divided into sub-genera, or
sections, or lesser groups. As Fries has well remarked, little groups of
species are generally clustered like satellites around other species. And
what are varieties but groups of forms, unequally related to each other,
and clustered round certain forms--that is, round their parent-species.
Undoubtedly there is one most important point of difference between
varieties and species, namely, that the amount of difference between
varieties, when compared with each other or with their parent-species, is
much less than that between the species of the same genus. But when we
come to discuss the principle, as I call it, of divergence of character, we
shall see how this may be explained, and how the lesser differences between
varieties tend to increase into the greater differences between species.
There is one other point which is worth notice. Varieties generally have
much restricted ranges. This statement is indeed scarcely more than a
truism, for if a variety were found to have a wider range than that of its
supposed parent-species, their denominations would be reversed. But there
is reason to believe that the species which are very closely allied to
other species, and in so far resemble varieties, often have much restricted
ranges. For instance, Mr. H.C. Watson has marked for me in the well-sifted
London catalogue of Plants (4th edition) sixty-three plants which are
therein ranked as species, but which he considers as so closely allied to
other species as to be of doubtful value: these sixty-three reputed
species range on an average over 6.9 of the provinces into which Mr. Watson
has divided Great Britain. Now, in this same catalogue, fifty-three
acknowledged varieties are recorded, and these range over 7.7 provinces;
whereas, the species to which these varieties belong range over 14.3
provinces. So that the acknowledged varieties have very nearly the same
restricted average range, as have the closely allied forms, marked for me
by Mr. Watson as doubtful species, but which are almost universally ranked
by British botanists as good and true species.
SUMMARY.
Finally, varieties cannot be distinguished from species--except, first, by
the discovery of intermediate linking forms; and, secondly, by a certain
indefinite amount of difference between them; for two forms, if differing
very little, are generally ranked as varieties, notwithstanding that they
cannot be closely connected; but the amount of difference considered
necessary to give to any two forms the rank of species cannot be defined.
In genera having more than the average number of species in any country,
the species of these genera have more than the average number of varieties.
In large genera the species are apt to be closely but unequally allied
together, forming little clusters round other species. Species very
closely allied to other species apparently have restricted ranges. In all
these respects the species of large genera present a strong analogy with
varieties. And we can clearly understand these analogies, if species once
existed as varieties, and thus originated; whereas, these analogies are
utterly inexplicable if species are independent creations.
We have also seen that it is the most flourishing or dominant species of
the larger genera within each class which on an average yield the greatest
number of varieties, and varieties, as we shall hereafter see, tend to
become converted into new and distinct species. Thus the larger genera
tend to become larger; and throughout nature the forms of life which are
now dominant tend to become still more dominant by leaving many modified
and dominant descendants. But, by steps hereafter to be explained, the
larger genera also tend to break up into smaller genera. And thus, the
forms of life throughout the universe become divided into groups
subordinate to groups.
|
