A species, in the strictly morphological sense, is simply an assemblage of individuals which agree with one another, and differ from the rest of the living world in the sum of their morphological characters; that is to say, in the structure and in the development of both sexes. If the sum of these characters in one group is represented by A, and that in another by A + n; the two are morphological species, whether n represents an important or an unimportant difference.
The great majority of species described in works on Systematic Zoology are merely morphological species. That is to say, one or more specimens of a kind of animal having been obtained, these specimens have been found to differ from any previously known by the character or characters n; and this difference constitutes the definition of the new species, and is all we really know about its distinctness.
But, in practice, the formation of specific groups is more or less qualified by considerations based upon what is known respecting variation. It is a matter of observation that progeny are never exactly like their parents, but present small and inconstant differences from them. Hence, when specific identity is predicated of a group of individuals, the meaning conveyed is not that they are all exactly alike, but only that their differences are so {292} small, and so inconstant, that they lie within the probable limits of individual variation.
Observation further acquaints us with the fact, that, sometimes, an individual member of a species may exhibit a more or less marked variation, which is propagated through all the offspring of that individual, and may even become intensified in them. And, in this manner, a variety, or race, is generated within the species; which variety, or race, if nothing were known respecting its origin, might have every claim to be regarded as a separate morphological species. The distinctive characters, of a race, however, are rarely equally well marked in all the members of the race. Thus suppose the species A to develop the race A + x; then the difference x is apt to be much less in some individuals than in others; so that, in a large suite of specimens, the interval between A + x and A will be filled up by a series of forms in which x gradually diminishes.
Finally, it is a matter of observation that modification of the physical conditions under which a species lives favours the development of varieties and races.
Hence, in the case of two specimens having respectively the characters A and A + n, although, primâ facie, they are of distinct species; yet if a large collection shows us that the interval between A and A + n is filled up by forms of A having traces of n, and forms of A + n in which n becomes less and less, then it will be {293} concluded that A and A + n are races of one species and not separate species. And this conclusion will be fortified if A and A + n occupy different stations in the same geographical area.
Even when no transitional forms between A and A + n are discoverable, if n is a small and unimportant difference, such as of average size, colour, or ornamentation, it may be fairly held that A and A + n are mere varieties; inasmuch as experience proves that such variations may take place comparatively suddenly; or the intermediate forms may have died out and thus the evidence of variation may have been effaced.
From what has been said it follows that the groups termed morphological species are provisional arrangements, expressive simply of the present state of our knowledge.
We call two groups species, if we know of no transitional forms between them, and if there is no reason to believe that the differences which they present are such as may arise in the ordinary course of variation. But it is impossible to say whether the progress of inquiry into the characters of any group of individuals may prove that what have hitherto been taken for mere varieties are distinct morphological species; or whether, on the contrary, it may prove that what have hitherto been regarded as distinct morphological species are mere varieties.
What has happened in the case of the crayfish is this: {294} the older observers lumped all the Western European forms which came under their notice under one species, Astacus fluviatilis; noting, more or less distinctly, the stone crayfish and the noble crayfish as races or varieties of that species. Later zoologists, comparing crayfishes together more critically, and finding that the stone crayfish is ordinarily markedly different from the noble crayfish, concluded that there were no transitional forms, and made the former into a distinct species, tacitly assuming that the differential characters are not such as could be produced by variation.