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 larvæ 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 Müller 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 antennæ 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, &c., 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 palæontology, 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 towards 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.