REDUCTION OF VARIABILITY. 123 



must necessarily lose its variability. Darwin and Weismann 

 have thought that, after isolation, a group could continue to 

 change by a heightening of its variability through natural se- 

 lection. The obvious reason for this idea lies in the fact, that in 

 their time the real nature of geno- variation of its cause was not 

 understood. The variability of an isolated group is limited, and 

 the smaller the group, the more limited its potential variabili- 

 ty, the sooner it will be pure altogether. If a few individuals of 

 a variable group stock an island the population will soon be 

 pure, and if two little colonies start on two islands, each island 

 may have its own local and pure species after very few genera- 

 tions. The fact that islands are frequently found to have spe- 

 cies of plants or animals which exist nowhere else, need not be 

 taken as proof for the adaptation of these species to the con- 

 ditions on those islands. To explain how all the individuals on 

 one island have come to be pure for one set of characters, we 

 need not ascribe any selection value to those characters. 



Weismann has built up a complicated structure of hypothe- 

 ses upon hypotheses about the mechanism of heredity. In his 

 last writings he assumed a reciprocal relation between the 

 determinants for an organ and this oigan itself, so that, if for 

 any reason some kind of determ nant would be better nourish- 

 ed, they would thus become stronger and get a relatively big- 

 ger share of the available food, so that they would become still 

 more numerous and stronger, and the organ in question would 

 be still better developed in the off spring of the modified indi- 

 viduals. This hypothesis is simply an attempt to explain the 

 way in which modifications, the effect of the environmental 

 factors of development could become transmittable. It is es- 

 sentially Lamarck's theory. If it were true, that occasionally 

 modifications were seen to be inherited in the way in which we 

 see special characters inherited in higher organisms, Weis- 

 mann's theory of germinal selection would become a plausible 

 explanation, but as the facts stand, the hypothesis is not justi- 

 fied by them. As we will see later, the theory, that the genes are 

 essentially chemical compounds with autokatalytic properties 



