402 BIOLOGY AND ITS MAKERS 



latter being united into ids, idanls, etc. The way in which 

 he assumes the interactions of these units gives to his theory 

 a highly S})ecuhiti\e character. The conception of the 

 complex organization of the germ-plasm which Weismann 

 reached on theoretical grounds is now being established on 

 the basis of observation (see Chapter XIV, p. 313). 



The Origin of Variations. — The way in which Weismann 

 accounts for the origin of variation among higher animals 

 is both ingenious and interesting. In all higher organisms 

 the sexes are separate, and the reproduction of their kind is 

 a sexual process. The germinal elements involved are seeds 

 and pollen, eggs and sperms. In animals the egg bears all 

 the hereditary qualities from the maternal side, and the 

 sperm those from the paternal side. The intimate mixture 

 of these in fertilization gives great possibilities of variations 

 arising from the different combinations and permutations of 

 the vital units within the germ-plasm. 



This union of two germ-plasms Weismann calls amphi- 

 mixis, and for a long time he maintained that the purpose 

 of sexual reproduction in nature is to give origin to varia- 

 tions. Later he extended his idea to include a selection, 

 mainly on the basis of nutrition, among the vital elements 

 composing the germ-plasm. This is germinal selection, 

 which aids in the production of variations. 



In The Evolution Theory, volume II, page 196, he says: 

 "Now that I understand these processes more clearly, my 

 opinion is that the roots of all heritable variation lie in the 

 germ-plasm; and, furthermore, that the determinants are 

 continually oscillating hither and thither in response to 

 very minute nutritive changes and are readily compelled 

 to variation in a definite direction, which may ultimately lead 

 to considerable variations in the structure of the species, if 

 they are favored by personal selection, or at least if they are 

 not suppressed by it as prejudicial." 



