Physiological Selection. 43 



reproductive systems ; but eventually, on account of 



independent variation, other differences would super- 

 vene, and the variety would take rank as a true 

 species. 



Now we must remember that physiological isola- 

 tion is not like those other forms of isolation (e.g. 

 geographical) which depend for their occurrence on 

 accidents of the environment, and which may therefore 

 take place suddenly in a full degree of complete- 

 ness throughout a large section of a species. Physio- 

 logical isolation depends upon distinctive characters 

 belonging to organisms themselves ; and it would 

 be opposed to the whole theory of descent with 

 progressive modification to imagine that absolute 

 sterility usually arises in a single generation between 

 two sections of a perfectly fertile species. Therefore 

 evolutionists must believe that in most, if not in all 

 cases — could we trace the history, say of any two 

 species, which having sprung from a single parent 

 stock on a common area, are now absolutely sterile 

 with one another we should find that this mutual 

 sterility had been itself a product of gradual evolution. 

 Starting from complete fertility within the limits of a 

 single parent species, the infertility between derivative 

 or divergent species, at whatever stage in their evolution 

 this began to occur, must usually at first have been well- 

 nigh imperceptible, and thenceforth have proceeded 

 to increase stage by stage. 



But, if it be true that physiological isolation between 

 genetically allied groups must usually itself have been 

 the product of a gradual evolution ; and if, when 

 fully evolved, it constitutes a condition of the first 

 importance to any further differentiation of these 



