396 Coe — A Century of Zoology in America. 



necessary to depend upon the somatic characters as evi- 

 dence of the hereditary constitution of an individual, it 

 now became possible, knowing the hereditary constitution 

 of the parents of any pair of individuals, to predict with 

 almost mathematical certainty the characters of their 

 possible offspring. 



In general, the laws of possible chance combinations of 

 any group of characters determine the probability of any 

 particular offspring possessing one or many of those 

 characters. The' physical basis for such Mendelian 

 inheritance is evidently the chance combinations of 

 chromosomes which result from the processes of matura- 

 tion and union of the germ cells. 



Certain limitations to the law are met with because 

 the relatively small number of chromosomes involves 

 linkage of genes, because of the occasional interchange of 

 groups of genes between homologous chromosomes, and 

 because the relative activity or potency of any partic- 

 ular gene may differ in different races, and, finally, 

 because the normal activity of any given gene may be 

 modified or inhibited by the action of other genes. It is 

 by no means certain, however, that all inheritance is 

 Mendelian, for there still remains much evidence that the 

 hereditary basis of certain characters may be resident in 

 the cytoplasm, rather than in the chromosomes. A 

 recent book by Morgan, Sturtevant, Muller and Bridges 

 (1915), entitled "the mechanism of Mendelian heredity" 

 gives the cytological explanation of Mendelian inher- 

 itance. 



Americans have from the first taken a leading part in 

 this field of research and have been quick to recognize its 

 practical applications to the improvement of breeds in 

 both animals and plants. This prominent position is 

 largely due to the experimental work of Castle, Daven- 

 port, Morgan, Jennings, Pearl, and their co-workers on 

 animals and that of East, Emerson, Davis, Hayes and 

 Shull on plants. 



The geneticist now realizes that the appearance of the 

 body (phenotype) gives but little clue to the inheritance 

 (genotype). That two white flowers produce only pur- 

 ple offspring, or two white fowls only deeply colored 

 chickens, or that a pair of guinea pigs, one of which is 

 black and the other white, have only gray agouti off- 

 spring, while other apparently similar white flowers or 



