74 EVOLUTION OF LIVING ORGANISMS. 



is brought about by the crossing of the new variety with 

 the original species with which it may now come into 

 contact. This kind of regression by hybridisation, as De 

 Vries has shown, is very difficult to prevent with plants 

 fertilised by wind or by insects. It leads to the forma- 

 tion of intermediate forms ; but the new hereditary con- 

 stitution can never be quite suppressed. True regression 

 is due to the tendency of the offspring of parents which 

 deviate from the mean to return to that mean of the 

 race ; it is the inevitable consequence of the interbreeding 

 of individuals endowed with unequal inheritance. If the 

 race is quite uniform, composed of homozygotes with the 

 same hereditary factors, there can naturally be no re- 

 gression ; this has been well established by numerous 

 and prolonged experiments. But if there is inequality of 

 inheritance ever so slight, regression will take place on 

 the cessation of selection. It is a universal phenomenon 

 common to all impure races, whether artificial or natural. 



In the foregoing chapter it has been shown that 

 natural selection acts by eliminating the unfit and so 

 leaving the fit to continue the race ; that this selection is 

 effective not only between widely divergent forms, but 

 also between individuals differing from one another 

 by ordinary variations ; and lastly, that the process of 

 natural selection is strictly analogous to that of artificial 

 selection practised by man. But some important points 

 still remain to be discussed. 



Are variations continuous or discontinuous is a ques- 

 tion which has given rise to much controversy. Con- 

 tinuity here means gradation of variation from one ex- 

 treme to another, so that, with regard to the measurement 

 of a particular character, the individuate of a race could 

 be arranged in a series leading gradually from those 

 having the character developed to its greatest extent to 

 those in which it is least developed. The more gradual 

 the transition, the more perfect the continuity, the more 

 even would be the curve formed by the ascending series. 



