Increase in Variability in One Direction. 17 



by selection is based, therefore, as far as the availal)le 

 data enable us to decide, on the existence of strains with 

 heritable but hitherto latent characters. Such races are 

 highly variable, and their existence is betrayed when they 

 first are met with, by trifling anomalies which however 

 can easily be worked up by selection. As a result they 

 rapidly deport from the type of the species but only be- 

 cause they approach their nezv type : and as soon as this 

 has been reached by isolation or exceeded by selection 

 it is just as difficult to effect any further improvement 

 as in ordinary improved races. These varieties cannot 

 be evoked at will; we have to wait till they chance to ap- 

 pear. Nor when once fully developed can we improve 

 them further. Nothing but chance — that is to say some 

 unknown factor — can as yet overstep these tivo limits] 

 selection can effect no more than the most transparent 

 illusion of any thing approaching complete control. 



