494 BIOLOGY AND HUMAN LIFE 



we have, we must give them the most favorable conditions for 

 growth and development. (3) We must try to find out how the 

 breeds or strains themselves can be improved. 



364. Improvement by selection. For centuries the people 

 who raise plants and animals have carefully watched their crops 

 or flocks to find the most desirable individuals to be the parents 

 of the succeeding crops or flocks. The best heads of wheat are 



selected to serve as seed 



— for the next year ; the 



ii^zz^zz^:z=izizizizzz:r~ best beans and the best 



~ potatoes are set aside as 



the progenitors of the 



ZI^ZZ^ZZZZZZZZZZZ^ZZZ— crops to come ; the 



Zi::^ZIIZIi:iIIIz:iI^^^II^^^—^ best milk cows are se- 



lected to be the mothers 



Fig. 208. Variation in physiological of calves ; the swiftest 



characteristics mares are the mothers 



Each line represents the relative amount of milk of ColtS ; and SO On. In 

 given by 16 cows in one month. The poorest yield ■, . ,-, "U i 



(represented by the shortest line) averaged 20 ^niS way tnere nas DCen 



pounds a day; the best cow averaged 30 pounds a a Steady improvement 



day. Not only did one cow differ from another, • fU <- 1- f /^ 



but for each cow the yield varied from day to day. ^^ ^ne StOCK 01 Clomes- 



In like manner, the percentage of fat in the milk tic plants and auimals 



varied from cow to cow; and for every cow, from ,.^. . ™,, 



day to day (FlgS. 2 10, 2Il). The 



plants cultivated in the 

 nineteenth century were better plants, from the farmer's point 

 of view, than those cultivated in the tenth century ; and so with 

 the animals cultivated at different times. 



Careful observers familiar with farm life and practice have 

 noted that cultivated races of plants and animals tend to de- 

 teriorate, or decline in quality, in a few generations unless the 

 selection is continued every year. The explanation of this was 

 not known. The only remedy is to select for every generation 

 the best or the most desirable individuals or seeds. 



There has been improvement, then, through artificial selec- 

 tion, but it has not insured the permanence of certain qualities. 

 Selection must be continuous. 



