is hardly possible to discover clear distinctive characters between
them.
Every naturalist who has had the misfortune to undertake the
description of a group of highly varying organisms, has encountered
cases (I speak after experience) precisely like that of man; and if of
a cautious disposition, he will end by uniting all the forms which
graduate into each other, under a single species; for he will say to
himself that he has no right to give names to objects which he cannot
define. Cases of this kind occur in the Order which includes man,
namely in certain genera of monkeys; whilst in other genera, as in
Cercopithecus, most of the species can be determined with certainty. In
the American genus Cebus, the various forms are ranked by some
naturalists as species, by others as mere geographical races. Now if
numerous specimens of Cebus were collected from all parts of South
America, and those forms which at present appear to be specifically
distinct, were found to graduate into each other by close steps, they
would usually be ranked as mere varieties or races; and this course has
been followed by most naturalists with respect to the races of man.
Nevertheless, it must be confessed that there are forms, at least in
the vegetable kingdom (19. Prof. Nageli has carefully described several
striking cases in his Botanische Mittheilungen, B. ii. 1866, ss.
294-369. Prof. Asa Gray has made analogous remarks on some intermediate
forms in the Compositae of N. America.), which we cannot avoid naming
as species, but which are connected together by numberless gradations,
independently of intercrossing.
Some naturalists have lately employed the term sub-species to
designate forms which possess many of the characteristics of true
species, but which hardly deserve so high a rank. Now if we reflect on
the weighty arguments above given, for raising the races of man to the
dignity of species, and the insuperable difficulties on the other side
in defining them, it seems that the term sub-species might here be
used with propriety. But from long habit the term race will perhaps
always be employed. The choice of terms is only so far important in
that it is desirable to use, as far as possible, the same terms for the
same degrees of difference. Unfortunately this can rarely be done: for
the larger genera generally include closely-allied forms, which can be
distinguished only with much difficulty, whilst the smaller genera
within the same family include forms that are perfectly distinct; yet
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