30 Breeding Plants and Animals. 



tion of natural species and to the artificial .evolution 

 of higher types of economic plants and animals? 



Books, monographs and bulletins reciting the re- 

 sults of experiments in those laws of life in which the 

 breeder is interested are also beginning to appear. To 

 illustrate this class of literature, and to show that 

 such experiments, theoretical studies and measurements 

 have value in the study of living things as well as in 

 working out the science of astronomy or mechanics, trie 

 following example is given: Quetelet found that the 

 law of error applies to living organisms as well as 

 to other phenomena. To illustrate this he used the 

 heights of soldiers, all natives of the one country and 

 district. If arranged in a row from the shortest to 

 the tallest and a line be drawn over their heads these 

 several things are observed: (a) The center man in 

 the line represents the average in height, (b) For 

 nearly the entire length the line is almost straight, but 

 descends slightly, (c) At the lower end the line curves 

 rapidly downward, (d) At the upper. end the line 

 curves rapidly upward. 



Yield of milk in dairy cows, speed in trotting horses, 

 strength in draft horses, the thickness of lean meat 

 on the breast of Asiatic fowls, men's capacity for music 

 and all other special and general characteristics seem 

 subject to representation by the same curved line. That 

 is, in the stable variety or species the many do not vary 

 greatly from the average, but there are a few which are 

 weak and a few which are strong in each quality. Thus 

 in case of wheat plants grown in hills 4x4 inches apart, 

 one plant in a hill, there are a few plants in a hundred 

 which are tall and a few which are short : some 

 are heavy yielders, others light yielders, and so 

 on. This expression of the law of variation does 

 not apply to visible qualities alone, but to all qualities. 

 Most important of all, it applies to variation itself. 

 Most species, varieties, families and individuals of ani- 



