CHAPTER IX. 



THE EFFECT OF FOOD AND OF PRODUCTS OF 

 METABOLISM. 



Effect of artificial manures on growth of crops Effect of nutrition 

 on plant variation Development of bees and of aphides in rela- 

 tion to food Influence of nature of food on wing markings of 

 certain Lepidoptera Dependence of colour of larvae on plant pig- 

 ments Influence of food on growth of tadpoles Plumage of cer- 

 tain birds altered by abnormal diet Quality of food influences 

 organs of digestion Every organism probably has specific metab- 

 olism, which has especially adverse action on its own growth 

 Products of metabolism may stimulate growth Effects of small 

 quantities of urea, uric acid, and ammonium salts Influence of 

 volume and of surface area of water on growth of pond snail 

 Influence of surface area on growth of tadpole Effects of increas- 

 ing quantities of metabolic products on characters of a snail, and 

 of a Crustacean. 



DARWIN records * that Andrew Knight was of the 

 opinion that " of all the causes which induce variability, 

 excess of food, whether or not changed in nature, is 

 probably the most powerful." Darwin himself, more- 

 over, was inclined to accept this view of the potency of 

 food as probable. That changes in the amount and the 

 quality of the food available for an organism during its 

 growth must of necessity exert an important influence 

 on the course of that growth, and presumably, there- 

 fore, on the final limits of its attainment, is sufficiently 

 obvious both from one's own everyday experience, and 



* " Animals and Plants," vol. ii. p. 244. 



