I04 ZOOLOGY 



growth of the body but the size and quality of the adult organ- 

 ism as well. Life would be impossible without food, oxygen, 

 water and suitable temperature. Any variation in these con- 

 ditions at once has its effect upon the organism. Experiment 

 shows that the varying degrees of salinity of the water may be 

 accompanied 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 the body of 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. Vari- 

 ations in organisms, which are due to use and disuse or to the 

 action of the environment, are known as fluctuations. As we 

 have seen, these fluctuations do not seem to be inherited. On 

 the other hand new qualities in organisms, which arise in the 

 germ plasm, are known as mutations and are transmissible. 



135. 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 directly. Only 

 a limited amount of vegetation can be supported by the earth 

 without cultivation. The number of animals therefore which 

 can find a livelihood on the earth is in turn restricted. There is, 

 however, no such limit of the powers of reproduction, either 

 among plants or animals. Any pair of organisms if unchecked 

 could in a very few years supply descendants enough to populate 

 the earth up to its full powers of support. That they do not 

 thus multiply at a geometric ratio is due solely to the influences 

 at work to destroy these descendants. Any group of organisms 



