Increase in Variability in One Direction. 17 
selection is based, therefore, as far as the available 
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 depart from tlie type of the species but only be- 
cause they approach their neiv type: and as soon as this 
has been reached bv isolation or exceeded bv selection 
j *f 
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 two limits; 
selection can effect no more than the most transparent 
illusion of any thing approaching complete control. 
