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! 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
those 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, as geographical races! Many years ago, when comparing, and seeing
others compare, the birds from the separate islands of the Galapagos
Archipelago, both 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 it cannot be doubted would 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 most 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 both as distinct species;
but what distance, it has been well asked, will suffice? if that between
America and Europe is ample, will that between the Continent and the
Azores, or Madeira, or the Canaries, or Ireland, be sufficient? It must be
admitted that many forms, considered by highly-competent judges as
varieties, have so perfectly the character of species that they are ranked
by other highly-competent judges as good and true species. But to discuss
whether they are rightly 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 on the attempt to determine their rank. I will here give
only a single instance,--the well-known one of the primrose and cowslip, or
Primula veris and elatior. These plants differ considerably in appearance;
they have a different flavour and emit a different odour; they flower at
slightly different periods; they grow in somewhat different stations; they
ascend mountains to different heights; they have different geographical
ranges; and lastly, according to very numerous experiments made during
several years by that most careful observer Gartner, they can be crossed
only with much difficulty. We could hardly wish for better evidence of the
two forms being specifically distinct. On the other hand, they are united
by many intermediate links, and it is very doubtful whether these links are
hybrids; and there is, as it seems to me, an overwhelming amount of
experimental evidence, showing that they descend from common parents, and
consequently must be ranked as varieties.
Close investigation, in most cases, will bring naturalists to an agreement
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 forms of doubtful
value. 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 attract
his attention, varieties of it will almost universally be found recorded.
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 very
generally considered as 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.
When a young naturalist commences the study of a group of organisms quite
unknown to him, he is at first much perplexed to determine what differences
to consider as specific, and what as varieties; 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 enabled to
make up his own mind which to call varieties and which species; 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,
moreover, he comes to study allied forms brought from countries not now
continuous, in which case he can hardly hope to find the intermediate links
between his doubtful forms, he will have 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 in 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 high 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 leading to more strongly marked and more
permanent varieties; and at these latter, as leading to sub-species, and to
species. The passage from one stage of difference to another and higher
stage may be, in some cases, due merely to the long-continued action of
different physical conditions in two different regions; but I have not much
faith in this view; and I attribute the passage of a variety, from a state
in which it differs very slightly from its parent to one in which it
differs more, to the action of natural selection in accumulating (as will
hereafter be more fully explained) differences of structure in certain
definite directions. Hence I believe a well-marked variety may be justly
called an incipient species; but whether this belief be justifiable must be
judged of by the general weight of the several facts and views given
throughout this work.
It need not be supposed that all varieties or incipient species necessarily
attain the rank of species. They may whilst in this incipient state 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. 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 have to 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
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, and for mere convenience sake.
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 my
future work the discussion of these difficulties, and the tables themselves
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.
Alph. De Candolle and others have shown that plants which have very wide
ranges generally present varieties; and this might have been expected, as
they become 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 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), often 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 over the world, 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, will still inherit
those advantages that enabled their parents to become dominant over their
compatriots.
If the plants inhabiting a country and described in any Flora be divided
into two equal masses, all those in the larger genera being placed on one
side, and all those in the smaller genera on the other side, a somewhat
larger number of the very common and much diffused or dominant species will
be found on the side of the larger genera. This, again, 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 large 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 have
generally 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 present 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 smallest genera, with from only one to four species,
are absolutely excluded from the tables. These facts are of plain
signification on the view that species are only strongly marked and
permanent varieties; for whenever 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 is the
case, 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 holds good.
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 in those cases in which 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 always confirm the
view. I have also consulted some sagacious and most 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
usual amount of difference.
Moreover, the species of the large genera are related to each other, in the
same manner as the varieties of any one species are related to each other.