VARIATION 



439 



It is quite impossible to say offhand, in any given case, 

 how much is due to inherited differences or breed, and how 

 much is due to "' fluctuations " produced by the environment. 

 But all farmers know that, 

 besides controlling the con- 

 ditions under which their 

 plants and animals develop, 

 they must also be careful to 

 select the right kinds of 

 seed or stock. The best 

 of care will not make an 

 ordinary white bean develop 

 into a plant bearing lima 

 beans, nor will extra feed- 

 ing make a scrub cow give 

 the kind of milk that may 

 be obtained from a good 

 Jersey cow. 



465. Improvement by 

 selection. All domestic 

 animals and plants have 

 been carefully watched for 

 centuries for the purpose 

 of selecting the most de- 

 sirable individuals as the 

 parents of the succeeding 

 crops or generations. The 

 best heads of wheat were 

 selected for seeding the 



following year ; the best beans and the best potatoes have 

 been set aside as the progenitors of the crops to come. And 

 the same principle has been applied in the raising of ani- 

 mals. The best milk cows were selected to be the mothers 

 of the calves, the swiftest mares were the mothers of the colts, 

 and so on. 



12 12.5 13 13.5 14 14.5 15 15.5 16 16.5 17 17.5 18 18.6 19 



Fig. 234. Variation in physiological 

 properties 



Forty thousand sugar beets, tested individually, 

 showed from 12 per cent to 19 per cent of 

 sugar. Beets containing 15.5 per cent of sugar 

 were the most frequent, but there were almost 

 as many beets with 15 per cent or with 

 16 per cent. As the percentage of sugar de- 

 parts more from the typical 15.5 per cent, the 

 number of individuals with a given sugar content 

 diminishes, so that the extremely poor and the 

 extremely rich beets are also fewest in number 



