INDIVIDUAL DIFFERENTIATION AND ADAPTATION. 95 



which variations are capable of being transmitted under the 

 influence of heredity. There is no known limit to the power 

 of organisms to vary. 



130. The Part Played by the Environment in Producing 

 Variation while not completely understood must be recognized 

 as very real. Even though much stress must be put upon the 

 hereditary complexity and instability of protoplasm as the 

 source of variations, it is evident that the external conditions 

 serve as stimuli to produce the changes on the inside. For 

 example, it is a matter of common observation that the quan- 

 tity and quality of food greatly influence not merely the rate 

 of growth but the size and quality of the adult organism as 

 well. Life would be impossible without food, oxygen, water 

 and suitable temperature. Any variation in these conditions at 

 once has its effect upon the organism. Experiment shows that 

 the varying degrees of salinity of the water may be accom- 

 panied by striking individual differences of form in certain 

 marine animals. Caterpillars of certain butterflies placed in 

 boxes lined with differently colored papers develop pupae with 

 colors harmonizing with those of the boxes containing them. 

 Colors in various animals are intensified or changed by special 

 foods or by changed temperature. In general it may be said 

 that changes in any of the conditions important to animal life 

 produce some change or variation in those animals subjected 

 thereto. Since this is true, it becomes inevitable that the 

 various individual animals on the earth are differentiated from 

 each other somewhat as was seen to be the case with the cells 

 and tissue of which the individual itself is composed. The 

 following paragraphs trace out some of the ways in which 

 this differentiation of individuals takes place, the relations of 

 the various organisms to each other and to the environment. 



131. The Struggle for Existence. All animals (with a 

 few possible exceptions in those which possess chlorophyll) 

 depend ultimately upon green plants for food, those which 

 live on other animals no less than those which use plant food 



