EXTERNAL INFLUENCES AS CAUSES OF VARIATION 225 



can, the particular modifying influence of each of the conditions 

 of life with which animals and plants are surrounded. In this way 

 we may get important information upon this most difficult and 

 unsatisfactory subject. Accordingly we undertake to ascertain 

 the effects due specifically to food, temperature, light, etc., the 

 elements that, taken together, constitute the conditions of life. 



SECTION II THE INFLUENCE OF FOOD UPON 

 VARIABILITY 



The best evidence goes to show that food affects develop- 

 ment both quantitatively and qualitatively. It is expedient to 

 consider the two separately. 



Quantitative effects of food. In general, as every stockman 

 knows, full feed means increased size, provided always there 

 has been no check during development. This is not only the 

 experience in the yards everywhere, but the world over the 

 largest animals are found on the best feeding grounds. Doubt- 

 less other external influences affect size, but certainly no other 

 equals the food supply, and if maximum development is expected 

 food must not be withheld, especially during the early stages of 

 growth. No amount of later feeding, after the individual has 

 accustomed itself to a reduced supply, can make amends for 

 early shortage. This is itself a deviation which easily becomes 

 permanent and follows the individual through life. 



Development, however, bears no direct ratio to food con- 

 sumed ; that is to say, the greater portion of all food is consumed 

 in supporting the vital processes, altogether without reference 

 to increase of weight or to labor performed. Under the best of 

 feeding we rarely recover 10 per cent of the food consumed in 

 the form of growth or increase of weight, and seldom realize as 

 much as one sixth in the form of labor or other output of the 

 body. The great mass of the food is either not digested at all 

 or goes to support the internal activities of the body, or else is 

 digested and passed out of the body without serving any useful 

 purpose whatever. 



It is a significant fact that stunted animals (and plants as well) 

 seldom recover from the evil effects of arrested development, 



