298 GENETICS 



individuals, and inferior individuals, all in many diverse 

 types. 



Thus it happens that in such an organism as man, or one 

 of the domesticated animals or plants, the individuals are 

 very different in their gene combinations, and consequently 

 the offspring usually have combinations very different from 

 those of their parents. They may therefore differ from their 

 parents in many of their characteristics. From vigorous and 

 efficient parents may be produced offspring that are weak 

 and inefficient. From defective parents may be produced off- 

 spring that are normal. Superior individuals are the result 

 of particularly fortunate combinations of genes, inferior 

 ones the result of unfortunate combinations. In organisms 

 living under natural conditions, on the other hand, much 

 less variety is produced through reproduction, because the 

 individuals that vary from the efficient types are quickly 

 destroyed by the severe conditions under which they live. 



Of the groups of organisms in which there is great diver- 

 sity of genes, the constitution of a human population is 

 typical. Such a population is made up of individuals, every 

 one of which differs genetically from every other (save in 

 the rare cases of identical twins). Every one is a different 

 combination from every other. Looking at the population 

 from the standpoint of vigor, efficiency and general fitness 

 for life and its functions, it might be divided roughly into 

 three groups. Forming a small percentage of all are the 

 superior individuals, those with particularly good combina- 

 tions of genes, their defective genes rendered harmless by 

 the presence of normal ones in the same pairs. At the other 

 extreme in the scale is the small group of the very defective, 

 the weak and inefficient, having poor combinations of genes, 

 their defective genes not accompanied by normal genes in 

 the same pairs; in man the paupers and criminals and de- 

 fectives. Between these two lies the large mediocre class, 

 composing perhaps 95 per cent of the population, having a 



