392 HEREDITY AND ENVIRONMENT 



fixes the type of a species. Finally the elimi- 

 nation of the unfit by natural selection is still 

 the only natural explanation of fitness, or 

 adaptation, in organisms. 



III. Methods of Modern Genetics 



1. Mendelian Association and Dissociation 

 of Characters. — Breeders have long known 

 that it is possible to get certain desirable char- 

 acters of an organism from one race and others 

 from another race. But since the discovery of 

 the Mendelian principles of heredity such new 

 combinations of old characters have been made 

 repeatedly, and with almost the same certainty 

 of results as when the chemist makes combina- 

 tions of elements or compounds. 



In Fig. 90, A and B, are shown two 

 guinea-pigs, one having long (L), rough and 

 tumbled (T), white (W) hair, and the other 

 having short (S), smooth (*S'm),red (R) hair. 

 When such animals are crossed one should get 

 in the F 2 generation 64 genotypes and 8 

 phenotypes, one of each of the latter being 

 homozygous and breeding true, as is shown in 

 Fig. 54 for trihybrid peas. These 8 pheno- 



