Species

Darwin and the Darwinians 179

In a short letter to Nature, while discussing whether variations in brain structure

could enable the evolution of instincts, Darwin noted in his own defense:

[t]he writer of the article in referring to my words “the preservation of useful varia-

tions of pre-existing instincts” adds “the question is, whence these variations?”

Nothing is more to be desired in natural history than that some one would be able

to answer such a query. But as far as our present subject is concerned, the writer

probably will admit that a multitude of variations have arisen, for instance in colour

and in the character of the hair, feathers, horns, &c., which are quite independent of

habit and of use in previous generations. It seems far from wonderful, considering the

complex conditions to which the whole organisation is exposed during the successive

stages of its development from the germ, that every part should be liable to occasional

modifications: the wonder indeed is that any two individuals of the same species are

at all closely alike.^86

So, in the end, Darwin proposed a “snowflake” theory of species—all members

are alike in some ways, but they are also unique individuals. Variation occurs natu-

rally, and it is weeded out according to how well suited it is to the conditions of life.

Species are held distinct incidentally to their being adapted to those conditions; they

are real at the time, although no rank seems to be absolute, and there is no particular

amount or kind of difference between them that marks out, or correlates with being,

distinct species. For sexual organisms all that can be said is that they do not, in

nature, interbreed (no matter whether they can be made to in captivity). He is not a

nominalist, as he notes in his response to Agassiz’s criticism,

[I]f species do not exist at all, as the supporters of the transmutation theory maintain,

how can they vary? And if individuals alone exist, how can differences which may be

observed among them prove the variability of species?^87

Darwin’s reply, to Asa Gray, was

I am surprised that Agassiz did not succeed in writing something better. How absurd

that logical quibble “if species do not exist how can they vary”? As if anyone doubted

their temporary existence?^88

We need to note with Darwin, as with others, that there is a difference between

denying that the rank of species has a definition, and denying that the term species

has one. Darwin denies the former, but not the latter. He is not a nominalist but a

pluralist with regard to what makes species distinct. Nevertheless, all these causes

resolve down to an aversion to interbreeding in sexual organisms and differences

in their sexual structures and constitutions, and selection maintaining the appropri-

ate forms and organs for living in the conditions in which they find themselves, for

asexuals. Of course, he also allowed that conditions of life may directly affect both

(^86) Da r win 1873.
(^87) Quoted in Lurie 1960, 29 7.
(^88) Quoted in Gayon 1996, 229; cf. also Ghiselin 1984, chapter 4.