90 



THE DOCTRINE OF DESCENT. 



individual species, the latter criterion alone has almost 

 always been employed, while the former has been en- 

 tirely neglected. Later, it is true, the genealogical idea 

 of the common descent of all individuals of each separate 

 species was supplemented by the physiological defini- 

 tion that all the individuals of every species are capable 

 of producing fertile offspring, by intercrossing, whereas 

 sexual intercourse between individuals of different species 

 produces only sterile offspring or none at all. In prac- 

 tice, however, it was considered quite enough if, among 

 a number of extremely similar animals under investiga- 

 tion, uniformity in all essential characters could be estab- 

 lished, and no inquiry was made whether these individuals 

 ascribed to the same species were actually of common 

 origin, and capable, by crossing, of producing fertile off- 

 spring. The physiological definition was no more ap- 

 plied in the practical discrimination of animal and 

 vegetal species, than was the pre-supposed common 

 descent from a single ancestral pair. On the other hand, 

 two closely allied forms were distinguished without 

 scruple as two different ' good species,' whenever in a 

 number of similar individuals examined a constant dif- 

 ference could be demonstrated, even though of a merely 

 subordinate character. Here, again, no pains were taken 

 to ascertain whether the two different series were not 

 really descended from common ancestors, and were really 

 capable of generating in conjunction only sterile hybrids, 

 if any." 



That this radical condemnation of the post-Linnaean 

 manufacture of species is not too severe, is shown by 

 one fact among others; that within the fraternity such 

 utter discord as to the limitations of species prevailed. 



