You can discover more about a person in an hour of play than in a year of conversation
Plato
Chapter 2: Variation Under Nature
By: Charles Darwin, 1859
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
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 distant 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 singularly,
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 a 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 the 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 amongst plants, several
genera of 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 amongst 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 the 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 probable 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 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 go 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, 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! Amongst 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 characterised 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 greater 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 wholly 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 steps 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-shell 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 consideration, 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 an
equally or 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
predominance, 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 be still 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 when 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 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) 63 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 63 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, 53 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 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 u into smaller genera. And thus, the forms of life throughout
the universe become divided into groups subordinate to groups.
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