Chapter 2 - Variation Under Nature
*
Variability *
Individual differences *
Doubtful species *
Wide ranging, much diffused, and common species vary most
*
Species of the larger genera in any country vary more 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 at all properly, a long
catalogue of dry facts should be given; but these I shall
reserve for my future work. Nor shall I here discuss the
various definitions which have been given of the term species.
No one definition has as yet 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 in one
part, either injurious to or not useful to the species,
and not generally propagated. 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 some few generations?
and in this case I presume that the form would be called
a variety.
Again, we have many slight differences which may be called
individual differences, such as are known frequently to
appear in the offspring from the same parents, or which
may be presumed to have thus arisen, from being frequently
observed in the individuals of the same species inhabiting
the same confined locality. No one supposes that all the
individuals of the same species are cast in the very same
mould. These individual differences are highly important
for us, as they afford materials for natural selection to
accumulate, in the same manner as man can accumulate 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 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. I should never have
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; I should have expected that
changes of this nature could have been effected only by
slow degrees: yet quite recently Mr 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 quite recently shown that the muscles in the larvae
of certain insects are very far from uniform. Authors sometimes
argue in a circle when they state that important organs
never vary; for these same authors practically rank that
character as important (as some few naturalists have honestly
confessed) which does not vary; and, under this point of
view, no instance of any important part varying will ever
be found: but under any other point of view many instances
assuredly can be given.
There is one point connected with individual differences,
which seems to me extremely perplexing: I refer to those
genera which have sometimes been called 'protean' or 'polymorphic,'
in which the species present an inordinate amount of variation;
and hardly two naturalists can agree which forms to rank
as species and which as varieties. We may instance Rubus,
Rosa, and Hieracium amongst plants, several genera of insects,
and several genera 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 some few exceptions, polymorphic in other countries,
and likewise, judging from Brachiopod shells, at former
periods of time. These facts seem to be 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 in these polymorphic genera variations in points
of structure 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
will be explained.
Those forms which possess in some considerable degree
the character of species, but which are so closely similar
to some 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 in their own country for a long
time; for as long, as far as we know, as have good and true
species. practically, when a naturalist can unite two forms
together by others having intermediate characters, 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 occur 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
links 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 judgement 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! 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 Gärtner, 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. These varieties,
moreover, will be often 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 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 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, 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. 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 certain 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 will tend to increase into
the greater differences between species.
There is one other point which seems to me 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 ought to be reversed.
But there is also reason to believe, that those 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 very
nearly the same restricted average range, as have those
very 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.
Finally, then, varieties have the same general characters
as species, for they cannot be distinguished from species,
except, firstly, by the discovery of intermediate linking
forms, and the occurrence of such links cannot affect the
actual characters of the forms which they connect; and except,
secondly, by a certain amount of difference, for two forms,
if differing very little, are generally ranked as varieties,
notwithstanding that intermediate linking forms have not
been discovered; but the amount of difference considered
necessary to give to two forms the rank of species is quite
indefinite. 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 certain species.
Species very closely allied to other species apparently
have restricted ranges. In all these several respects the
species of large genera present a strong analogy with varieties.
And we can clearly understand these analogies, if species
have once existed as varieties, and have thus originated:
whereas, these analogies are utterly inexplicable if each
species has been independently created.
We have, also, seen that it is the most flourishing and
dominant species of the larger genera which on an average
vary most; and varieties, as we shall hereafter see, tend
to become converted into new and distinct species. The larger
genera thus 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.
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