336 ELEMENTS OF BIOLOGY 



of the problems involved. The general principles of genetics have 

 been derived from experimental matings using as material a con- 

 siderable variety of plants and of animals. Particularly important 

 has been the study of inheritance in the fruit fly, Drosophila, and in 

 the guinea pig. The progeny of a single mating in Drosophila are 

 ordinarily several hundred. Guinea pigs reproduce quite rapidly, so 

 that a character can be followed through several generations in a 

 relatively short time. 



In order to work out with accuracy the behavior of an inheritable 

 character, a number of conditions are necessary that are wanting or 

 imperfect in Man. In the first place, it must be possible to control 

 matings at will in order to test the nature of the genes by appropri- 

 ate crosses. This is obviously impossible in human society. Secondly, 

 numerical ratios are reliable only when large numbers are involved, 

 and, as has been illustrated, one of the fundamental methods of 

 genetics is a statistical treatment of the data. The comparatively few 

 offspring in a human family do not afford ratios that permit definite 

 conclusions. Third, in order to determine the behavior of a genetic 

 character it must be studied through several generations. The 

 interval between generations in human families is approximately 

 two decades; to follow a character through more than two genera- 

 tions is usually beyond the opportunity of personal observation of 

 an investigator. Consequently, conclusions arrived at concerning 

 inheritance in Man are by inference from the studies of similar 

 characters in lower forms, or are derived from statistical studies 

 among populations and an occasional family history that has been 

 recorded with some accuracy. The study of human inheritance is 

 called eugenics; in the popular mind it is frequently distorted as a 

 sort of non-social program in some cases and over-socialization in 

 others. In general, the actual program looks toward collecting data 

 on human inheritance so that the facts may become known for 

 individual guidance or for assistance in the solution of medical, 

 legal, and social problems. 



