Species

192 Species

peculiarities, then all naturalists agree in calling them two species; that is what is

meant by the word species—that is to say, it is, for the practical naturalist, a mere ques-

tion of structural differences.[1]

...

[1] I lay stress here on the practical signication of “Species.” Whether a physiological

test between species exist or not, it is hardly ever applicable by the practical naturalist.^3

He repeats the requirement for the use of non-sexual characters in a later essay.^4
Huxley treats species in this limited taxonomic context^5 as merely conventional de-
nitional entities; that is, as a nominal essence. The note implies that he expects there
may be some real essence in the form of physiological (i.e., reproductive?) differ-
ences, but that they are not useable by practicing taxonomists. So long as there is
a constant unvarying character and a smallest diagnosable group, there is a spe-
cies. In his 1859 review of the Origin, when discussing Darwin’s views on sterility
of hybrids, Huxley asks what is known of the “essential properties of species.” He
summarizes:

Living beings, whether animals or plants, are divisible into multitudes of distinctly

deneable kinds, which are morphological species. They are also divisible into groups

of individuals, which breed freely together, tending to reproduce their like, and are

physiological species.^6

However, Huxley, following and in concert with Darwin, denied that reproduc-
tive isolation was sufcient to make a species, and noted that the degree of infertil-
ity between species varies from absolute to minor. Elsewhere, in his 1863 critical
review of the Origin,^7 he distinguishes between internal (“physiological”) and exter-
nal (“anatomical” or “morphological”) characterizations of species,^8 and in a paper
“Darwin on The Origin of Species” published in the Westminster Review in 1860 he
notes that naturalists employ the term species in a double sense to denote “two very
different orders of relations”:

When we call a group of animals, or of plants, a species, we may imply thereby either,

that all these animals and plants have some common peculiarity of form or structure;

or, we may mean that they possess some common functional character. That part of

biological science which deals with form and structure is called Morphology—that

which concerns itself with function, Physiology—so that we may conveniently speak

of these two senses or aspects of “species”—the one as morphological, the other as

physiological.^9

(^3) Huxley 1906, 226f.
(^4) Huxley 1906, 301ff.
(^5) This series of lectures was collected in 1864 and published in Huxley 1895, but the essay was rst
delivered in 1863.
(^6) Hux ley 1893, 50.
(^7) Reprinted in Huxley 1906 as chapter IX.
(^8) Forsdyke 2001, 31.
(^9) Huxley 1906, 302.