190 EVOLUTION AND DOGMA. 



reproductive system of most forms of life, and of the 

 intimate dependence of this system on the organism 

 to which it belongs, it appears a priori quite natural 

 that species or races, which in the beginning were 

 reciprocally fertile, should, in the course of time, 

 owing to some change in the conditions of existence, 

 or to protracted subjection to different sets of cir- 

 cumstances, become completely infertile. Many 

 causes have been assigned for this infecundity, but 

 the answers given are, it must be confessed, far 

 from satisfactory. " He who is able," says Darwin, 

 " to explain why the elephant, and a multitude of 

 other animals, are incapable of breeding when kept 

 under only partial confinement in their native coun- 

 try, will be able to explain the primary cause of 

 hybrids being so generally sterile. He will, at the 

 same time, be able to explain how it is that the races 

 of some of our domesticated animals, which have 

 often been subjected to new, and not uniform, con- 

 ditions, are quite fertile together, although they are 

 descended from distinct species which would prob- 

 ably have been sterile if originally crossed." 1 



True Significance of the Term " Species." 



From what precedes, then, it is manifest that 

 whether viewed from the standpoint of morphology, 

 or from that of physiology, species is something 

 which is extremely vague, and pregnant with diffi- 

 culties of all kinds. But it is also equally manifest 

 that the sterility of species, and of their hybrid prog- 

 eny, is something which establishes different groups 



1 Op. cit, p. 28. 



