1 66 Changing Plant and Animal Nature 



growth and development. Just as one cow may yield more 

 milk on some days than on others because of variations in 

 nutrition, one cow may yield more milk than another cow, 

 or one acre of corn may grow taller than another acre of 

 corn, because conditions are more favorable in one case than 

 in the other (Fig. 39) . We know from ordinary experience 

 that external conditions do influence growth and develop- 



FiG. 39. Nature and Nurture Both 



These two pigs are of the same breed and of the same age, and both were supplied 

 abundant food. The smaller one lived on a soil infested with round-worms and 

 became infected. Photograph by U. S. Bureau of Animal Industry. 



ment, and to that extent the variations found among indi- 

 viduals may be ascribed definitely to varying environmental 

 influences; and this conclusion is supported by experimental 

 evidence. 



Influence of Enviromnent 



We have already seen that the life of an individual plant 

 or animal is influenced from day to day, we might even 

 say from moment to moment, by fluctuations in external 

 conditions. These external conditions, from the very nature 

 of their fluctuations, work sometimes favorably and some- 



