PRACTICAL LAWS OF LIFE 



335 



higher than the worms and some insects have come to de- 

 pend upon it entirely. In sexual reproduction each indi- 

 vidual is built up by the mingling of the germinal elements 

 of two parents, and not only that, but of four grandparents, 

 eight great grandparents, and so on. This mingling, by pre- 

 potence of some characters and recession of others, causes 

 active variation, and this seems to be the chief purpose of 

 bisexual reproduction. By statistical analysis Galton proved 

 that an individual receives on the average 50 per cent of his 

 characters from his 

 parents, 25 per cent 

 from his grandpar- 

 ents, and the rest 

 from more remote an- 

 cestors. Given all the 

 forces of increase, va- 

 riation, and heredity, 

 another law comes into 

 play, the discovery of 

 which was Darwin's 

 great contribution. 



The law of natural selection. Nature selects the fittest to 

 survive. From the beginning, man has imitated nature in 

 selecting the plants and animals that suit his need or fancy, 

 and this is commonly distinguished as artificial selection. 

 Combination of these two processes has resulted in the spe- 

 cies and varieties, strains and breeds, that we now see in the 

 world. Progress has been made in the past chiefly by pick- 

 ing up chance variations as they have occurred in nature and 

 accidentally among domesticated plants and animals. Only 

 within recent years have we begun to learn how to select 

 the parents in order to cause desired variations. By eight 

 years of most accurate and painstaking experiments in cross- 

 ing and rearing varieties of garden peas, the Austrian monk, 



FIG. 164. Diagram illustrating Mendel's law 



of dihybrids, white being dominant and 



black recessive 



