476 



THE GENERAL PRINCIPLES OF ZOOLOGY 



recessive as we saw in the second generation, and hence these 

 mice will all be gray, but will produce gray and white offspring 

 in the proportion of three to one; since in the combination GW 

 the W is recessive, it is best written G{ W). This whole matter 

 may be represented graphically as follows : — 



ad infinitum. 





\G{W) 



11 



ad infinitum. 



Such is Mendel's law of the purity of the germ-cells manifested 

 through dominant and recessive characteristics ; it involves in 

 addition many complications, as when two individuals differ in 

 more than one characteristic and also when the first crossing: 

 results in individuals different from either parent, as sometimes 

 occurs. These matters will be found considered at length in 

 extended treatises on the subject. 



Hut the chief interest in the subject of heredity centers in the 

 characteristics which may be transmitted from the parent to 

 the offspring ; can the offspring inherit characters which the 

 parent may have acquired in the course of its existence through 

 the use or disuse of its various organs or through the direct 

 action of its environment, such as climate, food supply, and the 

 struggle with enemies ? In considering acquired characters we 

 should exclude mutilations, since these are brought about by 

 temporary not continued influences, and there is no authentic 

 evidence that they are ever inherited. Chinese women are not 

 born with deformed feet ; the Flathead Indians are not born 

 with deformed skulls ; the Jews are not born circumcised more 

 frequently than the Gentiles. A considerable number of living 

 biologists have pointed out the remarkable isolation, we max 

 even say insulation, of the germ-cells from the somatic cells. 



