50 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 
without 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-Linnzan 
manufacture of species is not too severe, is shown by 
one fact among others ; that within the fraternity such 
