420 THE CAUSES OF THE XT 



I don't care how slight and trivial, so long as it is 

 defined and constant, and does not depend on 

 sexual peculiarities, then all naturalists agree in 

 calling them two species ; that is what is meant 

 by the use of the word species that is to say, it 

 is, for the practical naturalist, a mere question of 

 structural differences. 1 



We have seen now to repeat this point once 

 more, and it is very essential that we should 

 rightly understand it we have seen that breeds, 

 known to have been derived from a common stock 

 by selection, may be as different in their structure 

 from the original stock as species may be distinct 

 from each other. 



But is the like true of the physiological charac- 

 teristics of animals ? Do the physiological differ- 

 ences of varieties amount in degree to those 

 observed between forms which naturalists call 

 distinct species ? This is a most important point 

 for us to consider. 



As regards the great majority of physiological 

 characteristics, there is no doubt that they are 

 capable of being developed, increased, and modi- 

 fied by selection. 



There is no doubt that breeds may be made as 

 different as species in many physiological charac- 

 ters. I have already pointed out to you very 



1 I lay stress here on the practical signification of "Species." 

 Whether a physiological test between species exist or not, it is 

 hardly ever applicable by the practical naturalist. 



