GENETICS IN RELATION TO HUMAN AFFAIRS 487 



tion may appear time after time, with many generations between 

 its recurrences, or in successive generations. Reversion of a 

 mutant race to the normal type, which may again produce the 

 mutant type, is also known. 



Some races seem more unstable with respect to change in genes 

 than others. Mutations are unknown in some experimental 

 forms, and frequent enough to be carefully studied in others. It 

 has been suggested that types of organisms may have cycles of 

 mutability at which times the germplasm is more unstable than at 

 others. Experiments on the induction of mutations by changes in 

 the environment of the individuals have shown that, in the indi- 

 vidual life cycle, some stages are more susceptible to abnormal 

 environmental conditions than others. Such experimental methods 

 as changes in temperature, moisture and food, and exposure to X- 

 rays and radium rays have ])een used. Knowledge of mutations 

 is rapidly increasing and their role in the evolution of organisms 

 seems very significant. 



Genetics in Relation to Human Affairs 



In man's utilization of natural resources, knowledge of inheri- 

 tance has been a useful tool. New varieties of plants and animals 

 have been established b}^ hybridization and continued selection. 

 Early-bearing plants, or plants otherwise adapted for particular 

 environments, have been isolated as pure Hues from heterogeneous 

 populations. The breeding of the types of cattle best suited for 

 beef, milk, or butter-fat production, and of horses for speed or 

 draft purposes are examples of the apphcation of INIendelian prin- 

 ciples (c/. Figs. 281-284). The preservation of mutations in 

 establishing new races has been accomplished in the case of the 

 short-legged Ancon sheep, polled Hereford cattle, and certain 

 plants. Genctical information is of the utmost value in the better- 

 ment of the animals and plants domesticated by man, and in the 

 estabhshment of new kinds of animals and plants for food or other 

 purposes. 



Man's characteristics are inherited from generation to genera- 

 tion as in the case of other living organisms. The course of hered- 

 ity is well understood in many cases and much information has been 

 accumulated. In human eye-color, the presence of a pigment on 

 the back of the iris gives rise to all shades of blue eyes. In addi- 



