VII 
ON THE INFERTILITY OF CROSSES 
167 
law existed was never more than a plausible generalisa¬ 
tion, founded on a few inconclusive facts derived from 
domesticated animals and cultivated plants. The facts were, 
and still are, inconclusive for several reasons. They are 
founded, primarily, on what occurs among animals in 
domestication; and it has been shown that domestication 
both tends to increase fertility, and was itself rendered 
possible by the fertility of those particular species being little 
affected by changed conditions. The exceptional fertility of 
all the varieties of domesticated animals does not prove that 
a similar fertility exists among natural varieties. In the next 
place, the generalisation is founded on too remote crosses, as in 
the case of the horse and the ass, the two most distinct and 
widely separated species of the genus Equus, so distinct indeed 
that they have been held by some naturalists to form distinct 
genera. Crosses between the two species of zebra, or even 
between the zebra and the quagga, or the quagga and the ass, 
might have led to a very different result. Again, in pre- 
Darwinian times it was so universally the practice to argue in 
a circle, and declare that the fertility of the offspring of a 
cross proved the identity of species of the parents, that experi¬ 
ments in hybridity were usually made between very remote 
species and even between species of different genera, to avoid 
the possibility of the reply : “ They are both really the same 
species and the sterility of the hybrid offspring of such 
remote crosses of course served to strengthen the popular 
belief. 
Now that we have arrived at a different standpoint, and 
look upon a species, not as a distinct entity due to special 
creation, but as an assemblage of individuals which have become 
somewhat modified in structure, form, and constitution so as 
to adapt them to slightly different conditions of life; which 
can be differentiated from other allied assemblages ; which 
reproduce their like, and which usually breed together—we 
require a fresh set of experiments calculated to determine the 
matter of fact,—whether such species crossed with their near 
allies do always produce offspring which are more or less 
sterile inter se. Ample materials for such experiments exist, 
in the numerous “ representative species ” inhabiting distinct 
areas on a continent or different islands of a group; or even 
