No. 613] EXPERIMENTS CONCERNING EVOLUTION 47 



ent milieus is the life-cycle (in a broad sense). The visi- 

 ble distinctive characters of the races— aside from addi- 

 tional mutations of a non-selective nature— are nothing 

 but the products of reaction of different types of metabo- 

 lism, allied with the different time relations of the cycle. 

 The method of the formation of geographic races in this 

 case must, therefore, be the following. The first con- 

 quest of a new territory is of course only possible wheu 

 the animal is preadapted, along general lines, to the new 

 medium. But that it can maintain itself depends upon its 

 power of special adaptation. The gypsy-moth, for exam- 

 ple, has repeatedly been brought into England, but it has 

 never established itself there. In the case of this form 

 the special adaptation means the coincidence, in the first 

 place, of the life-cycle with the seasonal cycle in nature. 

 And it is here that all the discriminating effect of selec- 

 tion comes in. The quantitative changes of the genes 

 which cause the time relations of the cycle are then the 

 material for selection, and selection acts according to Dar- 

 winian principles until the equilibrium is established. 

 Thus the genetic study of the quantitative changes of the 

 gene reveals anew the truth of Darwin's conception. 

 Furthermore, we see here how sterility of hybrids or com- 

 plete incompatibility of new forms may arise. We have 

 proved that the quantitative differences of the sex- factors, 

 which are themselves nothing but adaptations to the time- 

 relations of the cycle, are among the characteristic differ- 

 ences of these races. 4 There are, moreover, responsible 

 for the incompatibility in regard to sex which results in 

 intersexuality after crossing. Changes of exactly the 

 same type may easily make any cross-breeding impossi- 

 ble, since no organism can develop unless all the processes 

 of differentiation are coordinated in respect to their ve- 

 locity. Here we see, finally, why geographical races are 

 so often uniform and are characterized by certain traits of 



