- 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. |