GENETICS 239 



the same as it is at present. That method is selection. Those animals 

 and plants which were most valuable were selected for breeding, in the 

 belief that their good qualities would be transmitted. If even only a few 

 of these characteristics were inherited, long-continued selection would 

 result in great improvement. 



The discovery of Mendel's principles thus found mankind already in 

 possession of very valuable varieties of animals and plants. Man had 

 attained this result without knowing very much about how he did it. 

 Improvement has, of course, gone on since then. It is now considerably 

 plainer why certain results are obtained, and these results often come 

 more quickly. Among the important domestic animals, poultry have 

 probably yielded more to the newer Mendelian knowledge than any 

 others. Considerably less has been done with pigs and sheep, and little 

 has been revealed about Mendelian behavior of the valuable characters of 

 cattle and horses. Undoubtedly the cost of experimenting with these 

 larger animals and the long time involved, when one generation requires 

 several years, are responsible for the lag of knowledge concerning their 

 heredity. 



Plants have revealed more of their hereditary constitution, partly 

 because they are inexpensive to rear, partly perhaps because they are 

 of simpler composition. The most important feature of most crops is 

 yield, which is inherited, since varieties differ greatly in this respect. The 

 principal factor contributing to yield which is being studied now more 

 successfully than a generation ago is resistance to disease. The various 

 bacterial and fungous diseases of the grains and fruits are receiving con- 

 centrated attention at most of the experiment stations, and the results 

 attained are very gratifying. 



Room exists for improvement of man himself, through the elimination 

 or diminution of some of his defects. Every system of organs and every 

 sense organ exhibits hereditary deficiencies in some individuals, such as 

 feeble-mindedness, fragility of bones, a tendency to bleed, cataract of 

 the eyes, atrophy of muscles, and baldness. Some of these defects are 

 more important than others, but there is not one which the human race 

 would not choose to banish if it could. The only method is to avoid 

 reproduction by individuals possessing genes for the undesirable qualities. 

 With respect to most defects, this avoidance must be voluntary, and it is 

 uncertain how seriously men and women take their responsibilities. 

 Some of the more serious defects, such as feeble-mindedness and epilepsy, 

 are, however, frequently dealt with by law. At present 29 states of 

 the United States have laws designed to prevent people afflicted with 

 these infirmities from rearing families. 



Theoretically, man should be able to improve himself by favoring 

 those qualities, talents of various sorts, which it is particularly desirable 



