MORE ABOUT SPECIES
I wrote in the last chapter of the recognition of that degree of
"likeness" or kinship in animals and plants which we point to by the
word "species," and of the grouping of several similar species to
form a "genus," and of several genera to form a family, of families
to form orders, and of orders to form classes–and of the giving of
names to all these groups. Whilst the making of this or that lot of
species into a distinct _genus_, and giving it a new name is a mere
matter of convenience for the indication of more or less important
agreements and divergences, and is to a large extent arbitrary or an
expression of opinion–it has always been recognized among naturalists
that the group called "a species" is not a mere convention, but has
a real natural limitation. It is true that the actual things which
we see in studying natural history are so many units or individuals.
But the possibility of arranging these by pattern, colour and shape
into ultimate companies of which all the units are alike and differ
from all the units of another company, has been regarded as a natural
fact of primary importance and not a mere convention or convenience.
The conception of the "naturalness" of a species depends really upon
a further qualification of great importance as to what we naturalists
understand by it.
We find by rearing plants from seed and by causing animals to breed
under actual observation that the individuals of a species pair
with one another, and not with individuals of other species, and
further, that the young which they produce are like the parents–show
themselves, in fact, to be of the same "species." The species
continually year after year reproduces itself with little variation,
though some variation does occur. The faculty of pairing only within
the group, of never naturally breeding with members of other groups,
has accordingly been adopted as a test of species. Hybrids between
two species do not occur, except in very rare cases, in the state of
nature. It is not always the case that the members of two species
cannot possibly pair together, but it is the fact that they do not do
so. Man sometimes brings about such crossing or hybridization, and
it is a curious fact that the hybrids are often infertile or give
rise only to weakly offspring, which could not survive in the natural
struggle for existence. Sometimes, however, when the two hybridized
species happen to come from regions of the world remote from one
another, the resulting hybrids establish a vigorous race. There are
real obstacles (of which I will say more below) in natural conditions
to hybrid-breeding between any two species which occur naturally in
the same territory. Thus the idea of a species is expanded so as to
be not merely "a group of individuals of constant likeness in form
and characteristics," but we add to that definition a living or
constitutional quality expressed by the words, "which produce fertile
offspring by pairing with one another, but do not pair with the members
of other species."
This enables us to distinguish the conception of a "species" from that
of a "variety" or a "race." We find occasionally peculiarly-marked
examples of a species of plant or animal, or even local races of
peculiar form; but we do not regard them as "distinct species" if
we find that they breed as a rule with the ordinary members of the
species. The decisive test is the breeding. If the variety is found
not to breed with the regular species, but to keep apart and breed
only with other individuals like itself, then we say, "This is no mere
variety! It is a distinct species!" Unfortunately we have vast series
of animals, insects, and others, from all parts of the world, collected
and preserved in our museums, of which we know only the dead preserved
specimens. So that we cannot be sure in doubtful cases whether a series
of forms differing a little from the ordinary members of a species
indicate distinct species, as defined and tested by breeding. We have
in such a case to note the difference, and record it either as a
variety or as a species by a guess at the probabilities one way or the
other. Naturalists really _intend_ by the word "species" to designate
a form represented by numerous like individuals, which, in the
present natural conditions of the region they inhabit, have attained
a certain "stability" of distinctive form and character (not without
some variability within definite limits) and constitute a more or less
widely distributed population, the members of which inter-breed but do
not produce offspring with other allied species.
A good case by which to exhibit further our conception of a species is
that afforded by the species which are united in the genus Equus–the
horse-genus. There are living at the present day several wild kinds
of Equus–namely, the wild horse, or Tarpan, of the Gobi desert of
Mongolia, called after the Russian explorer Przewalski; two kinds of
Asiatic wild ass, called the Kiang and the Onegar; the African wild
ass, and two or three kinds of zebra. There are, besides, many kinds
of domesticated horses, ranging from the Shetland pony to the Flemish
dray horse, and from the Shire horse to the Arab. Then there are many
kinds of fossil extinct horses known, some of which clearly must be
placed in the genus Equus with the living kinds; others which have to
be separated into special genera (Hippidium, Onohippidium, etc.). Now,
as to the living forms or form-kinds of the genus Equus–which are we
to regard as true species, and which are only varieties and races of
lower significance than species? The answer is clear enough in regard
to several of them. The wild Mongolian horse and all the domesticated
horses are varieties, races, or breeds of one species, judged not only
by such marks as the possession of callosities on both the hind and
the fore legs, but also by the test of breeding. They breed together
and produce persisting races. But the asses and the zebras, though
they will form mules with the horse, do not in a state of nature
freely breed with it. When an ass or zebra is mated by man with the
horse it will produce hybrids, called "Mules," but will not in "a
state of nature" _establish_ a hybrid race. The asses and the zebras
are distinct from the horse, not only in markings and certain details
of shape and hair, but in the fact that they cannot be fused into one
race with him. There are no sufficient experiments on the aloofness
of zebras and asses from one another in regard to breeding, although
it seems that they cannot establish a mixed race, and are, therefore,
distinct species judged by that test as well as by their form and
marking. It is not known whether the so-called species of wild ass–the
Asiatic and the African–would prove to produce fertile or infertile
mules if intercrossed, nor has the test been applied to the very
differently-marked local races of the African zebras–Grevy's zebra,
Burchell's zebra, and the mountain zebra. It is likely enough that the
three or more "species" distinguished among zebras on account of their
being differently striped, and existing in different localities, would
be found to breed freely together, and prove themselves thus to be
entitled to be regarded as local "varieties" or "races," but not as
fully-separated true species.
Thus one sees how difficult it is to have knowledge of the breeding
test, even in regard to large animals. It is obvious that the
difficulty of obtaining it in regard to the thousands of kinds of
minute creatures is much greater. Yet when they say, "This is a
distinct species," naturalists do mean that it is not only marked
off from other animals or plants most like to it by a certain shape,
colour, or other quality or qualities, but that it breeds apart with
its own kind and does not naturally hybridize with those other forms
most like to it.
Although the kind of naturalist called a "systematist" who makes
it his business to accurately describe and record and distinguish
from one another all the existing species of some one group–say, of
antelopes, of mice, of flowering plants, of fishes, or of fleas–has
only a knowledge in a few instances of the breeding of the organisms
which he describes as "distinct species," he yet does know, in regard
to some one or more of his species in most groups, the facts of pairing
and reproduction, and what are the limits of variation in the markings
and other characteristics of at least one or two species definitely
submitted to the "breeding test," that is to say, ascertained to be
"true physiological species," kept apart by deep-seated chemical
differences in their blood and tissues. Hence it is legitimate for
him, by careful balancing and consideration of all the facts, to
determine–not absolutely, but by analogy–the value to be assigned
(whether as indicating true species or merely varieties capable
of pairing with the main stock) to points of difference among the
specimens of a dead collection brought from some distant land or from
some position in which it would be impossible to make observations
with regard to "pairing" and "breeding true."
Some 400 species of fleas have been described, and we are certain as
to the value of the characters relied on to distinguish those species,
owing to what we know of the breeding of some common species of fleas.
The flea of the domestic fowl, that of the domestic pigeon, that from
the house-martin, and that from the sand-martin–used to be considered
as one species until they were carefully examined twenty years ago. In
reality each of them has its own peculiar "marks," and they do not mix
with one another. The nests of the sand-martin yield only one species
of flea, namely that peculiar to the sand-martin. The hen-house,
the dove-cote, and the nests of the house-martin yield each their
flea maggots, which can be reared and become in each case a distinct
species with definite recognizable "characters." On the other hand,
the flea of the rabbit gives an opportunity of studying the limits of
variation in a "good" species. Rabbit warrens swarm with the rabbit
flea, and often a great number are found on one rabbit, the individual
fleas "varying"–"differing" from one another to a slight extent. The
"systematist" thus gets to know what organs are variable within the
limits of an undoubted physiological species of flea, and what are
comparatively constant–so that he can form a reasonable opinion about
the claim of other specimens which he may receive without full history
of their habits, to be regarded as true distinct species.
The fact that most important chemical differences of the blood and
digestive juices often accompany the small external differences
which enable us to distinguish one species of animal or plant from
another, makes it obvious that the knowledge of species is a very
valuable and necessary thing. One species of flea, the Pulex Cheopis,
habitually carries the plague bacillus from animals to man, and is
a cause of death; other species, extremely like it in appearance,
but distinguishable by a trained observer, do not carry the plague
bacillus, but if they swallow it, destroy it by digestion. One species
of gnat, the common grey gnat, digests and destroys malaria germs when
it sucks them up with blood; in an allied species, the spot-winged gnat
or Anopheles, the chemical juices of the gut allow the germ to live in
it and multiply, and so to be carried to men by the gnat's bite. So
with many other flies and parasites the recognition of the dangerous
species is of vital importance, and that recognition often depends on
minute features of form and colour not at once obvious to an ordinary
observer.
But this recognition of distinct species is, from the point of view of
the study of Nature, only a preliminary to the question, "How did these
species come about? How is it that there are so many species, some very
like one another, forming genera, and these genera grouped into related
families, these into larger groups, and so on, like the branches of
a family tree?" The answer to these questions given by Linnæus was:
"There are just so many species as the Infinite Being created at the
beginning of things, and they have continued to propagate themselves
unchanged ever since." The answer which we give to-day is that the
appearance of a huge family tree which our classification of animals
takes is due to the simple fact that it really is neither more nor less
than a family tree or pedigree–the "tree of life," of which the green
leaves and buds are the existing species. Further, we hold that the
existing species of a genus have "come into existence" by natural birth
from one ancestral species, its offspring having slightly varied (we
are all familiar with this individual variation in our own species, in
dogs, cats, trees, and shrubs), and that the varieties have wandered
apart and become continuously emphasized and selected for survival by
their fitness or suitability to the changed conditions around each of
them. Meanwhile a natural destruction, or failure of intermediate forms
to survive, has gone on.