Species

184 Species

can be differentiated from allied assemblages; which reproduce their like; which usu-

ally breed together; and, perhaps, when crossed with their near allies, always produce

offspring which are more or less sterile inter se.^108

Earlier, in his Contributions to the Theory of Natural Selection,^109 while discuss-
ing the numbers of species in the Malayan Archipelago of Papilionidae (a group
of butterflies) he discussed the principle by which he should give them the specific
rank. He noted,

One of the best and most orthodox definitions is that of Pritchard [sic],^110 the great eth-

nologist, who says, that “separate origin and distinctness of race, evinced by a constant

transmission of some characteristic peculiarity of organization” constitutes a species.

Now leaving out the question of “origin” which we cannot determine, and taking only

the proof of separate origin, “the constant transmission of some characteristic pecu-

liarity of organization,” we have a definition which will compel us to neglect altogether

the amount of difference between any two forms, and to consider only whether the

differences that present themselves are permanent. The rule, therefore, I have endeav-

oured to adopt is, that when the difference between two forms inhabiting separate

areas seems quite constant, when it can be defined in words, and when it is not confined

to a single peculiarity only, I have considered such forms to be species.

This is a generative conception again. He then discusses variation, polymor-

phisms, local varieties, co-existing varieties, and sub-species, and says,

Species are merely those strongly marked races or local forms which when in contact

do not intermix, and when inhabiting distinct areas are generally believed to have had

a separate origin, and to be incapable of producing a fertile hybrid offspring. ... it will

be evident that we have no means whatever of distinguishing so-called “true species”

from the several modes of variation here pointed out, and into which they so often pass

by an insensible gradation.^111

Wallace is taking Darwin’s approach to its natural conclusion, as it were. If variet-

ies are incipient species, and species merely well-marked and more or less perma-

nent varieties, then there is nothing sui generis about species as a categorical rank. In

this he was followed also by August Weismann (1834–1914),^112 who treated species

entirely as complexes of adaptations. Discussing a number of examples of series of

forms that can only be arbitrarily delimited, Weismann notes,

All the individual members of these series are connected by intermediate forms in

such a manner that a long period of constancy of forms seems to be succeeded by a

shorter period of transformation, from which again a relatively constant form arises.

(^108) Wallace 1889, 167.
(^109) Wallace 1870, 142.
(^110) Wallace is referring to James Prichard [1813, Book II, chapter 1; 3rd edition quoted, 1836]. See
above.
(^111) Wallace 1870, 161.
(^112) Weismann 1904.