EXTERNAL INFLUENCES AS CAUSES OF VARIATION 227 



sterility, and this result is hastened if the food contains unusual 

 proportions of carbohydrates, especially sugar. 



Effect of feed upon variability. It is a common belief that 

 plants and animals are more variable when well fed than other- 

 wise. Doubtless this is true, so far at least as appearances go, 

 for only under such favorable conditions are all the faculties 

 that are born into the exceptional individual able to develop and 

 become visible. When a race is living under mediocre conditions 

 there is a dead level in development. The mediocre individuals 

 have relatively the best chance, and few, will rise above the con- 

 dition of mediocrity. 



Whether full feed is a direct stimulant to variability or only 

 brings potential differences to the surface is therefore an open 

 question. The procedure indicated is, however, in either case the 

 same ; namely, to provide maximum conditions if the breeder 

 expects to realize the utmost from his best individuals or hopes 

 ever to find variations worth preserving. 



Herein lies the fact that well-bred animals often require more 

 feed than their scrub relatives. It was upon that point that they 

 departed from their kind not that they contracted to exist on 

 less feed, but that they were able to handle more feed and put 

 it to good use. If the purpose of the breeder were to develop 

 races with a minimum maintenance ration, it could be done; but 

 we keep domestic animals not for their society but for what they 

 can do, for what they can manufacture out of corn, oats, and 

 hay. We improve crops, not to see upon how poor land they may 

 live, but rather to increase their ability to construct valuable food 

 materials from the mineral elements of the soil and the inorganic 

 constituents of the atmosphere. Not minimum of consumption 

 but economic consumption is therefore the virtue sought. 



Evil effects of overfeeding. The plant will seldom suffer from 

 abundance of food, although it is not impossible. Excess of 

 nitrogen causes rank growth of stem, but phosphorus is needed 

 for seed. Something akin to a balanced ration is doubtless best 

 for plant as well as animal, yet the former has more of selective 

 ability than the latter. 



The animal is easily overfed, and if so the injury is likely to 

 be permanent. It results (i) in disorders of the digestive tract ; 





