44 COSMIC FRILOSOPEY, [pt. ii. 



oetween the morpLological and the physiological definitions 

 of species, and is usually known as the argument from the 

 infertility of hybrids. As ordinarily stated, indeed, this 

 argument is merely the expression of a sorry confusion oi 

 ideas. By a curious misunderstanding the infertility of the 

 mule is often urged as a direct objection to the Darwinian 

 theory. But this is putting the cart before the horse. It is 

 not the infertility of the offspring of tlie horse and the ass 

 which should be cited as an obstacle to the theory of natural 

 selection, but it is the fertility of the offspring of the carrier- 

 pigeon and the pouter, or of the pouter and tumbler. Mor- 

 phologically the carrier, the pouter, and the tumbler may 

 well be regarded as distinct species artificially developed 

 from a common wild stock; but so long as mutual infer- 

 tility is held to be the physiological test by which we are 

 to distinguish between varieties and species, it may be argued 

 that, in spite of their great morphological differences, the 

 carrier and the tumbler are only varieties and not true 

 species. And going a step farther, it may be argued that 

 until the theory of natural selection has accounted for the 

 rise of infertility between races descended from a common 

 stock, it has not completely performed the task of reconciling 

 deduction with observation. 



Against the derivation theory in general, this objection has 

 no weight whatever. That races originally fertile together 

 should, after long subjection to different sets of circumstances, 

 become infertile with one another, is a priori in the highest 

 degree probable, when we reflect upon the extreme sensi- 

 tiveness of the reproductive system to changes of habit in 

 the organism as a whole. When we remember that "the 

 constitution of many wild animals is so altered by confine- 

 ment that they will not breed even with their own females," 

 we need not be surprised that the leopard and the lion, 

 wliich during many ages have had very different habits of 

 life, will not breed with each other. Nor need we wondei 



