210 THE BREEDING OF ANIMALS 



a support to the doctrine of the inheritance of acquired 

 characters/' 1 



198. Causes of variation. The nature of variation 

 is still obscure. The fundamental causes are not easily 

 determined. " Our ignorance of the laws of variation 

 is profound," says Darwin. 



The results of the investigations in cytology have given 

 a more reasonable basis for understanding the subject 

 of variation, but it has not yet given us a wholly satis- 

 factory knowledge of the causes of variation. Bateson 

 holds that we are yet far from a satisfactory explanation 

 of the real nature of variation. He has concluded that 

 " Inquiry into the causes of variation is, in my judgment, 

 premature." We are, however, able to recognize and 

 classify certain apparent causes of variation. Such clas- 

 sification recognizes causes of variation as external and 

 internal. 



Davenport 2 has further classified the internal causes 

 of variation as: " 1. Internal influences affecting pri- 

 marily the individual, and 2. internal influences affecting 

 the race as a whole." 



199. Cell division a cause of variation. Every animal 

 is the product of the union of the germ substance of two 

 other animals. The union of the germ-cells is a union 

 of the characters of the parents. This combination of 

 the germinal matter of the two parents results in a rear- 

 rangement of some of the characters, and these may vary 

 materially from the original characters of the parents. 

 Weismann calls this mixing of the germ-plasm amphimixis. 

 He is of the opinion that sexual reproduction by cell union 

 and cell division is nature's plan for increasing variation. 



1 Thomson, " Heredity," pp. 230-236. 



2 Davenport, "Principles of Breeding," p. 155. 



