)0 THE DOCTRINE OF DESCENT. 



the idea of species, in the discrimination and nomencla- 

 ture of the individual species, the latter criterion alone 

 has almost always been employed, while the former has 

 been entirely neglected. Later, it is true, the genea- 

 logical idea of the common descent of all individuals of 

 each separate species was supplemented by the physio- 

 logical definition that all the individuals of every species 

 are capable of producing fertile offspring, by intercross- 

 ing, whereas sexual intercourse between individuals of 

 different species produces only sterile offspring or none at 

 all. In practice, however, it was considered quite enough 

 if, among a number of extremely similar animals under 

 investigation, uniformity in all essential characters could 

 be established, and no inquiry was made whether these 

 individuals ascribed to the same species were actually 

 of common origin, and capable, by crossing, of pro- 

 ducing fertile offspring. The physiological definition 

 was no more applied 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 

 v/ithout scruple as two different 'good species,' when- 

 ever in a number of similar individuals examined a con- 

 stant difference 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 



