DIFFERENTIATION OF INDIVIDUALS AND ADAPTATION 103 



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. 



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 

 will hold its own when, on an average, a pair of individuals can in 

 a lifetime bring to maturity another pair to take their place. 

 More than this means conquest of new territory; less than this, 

 the extinction of the group. When we recall that all organisms 

 have this unlimited power of reproduction, it is easy to see that a 

 time must soon come when a struggle for food and a foothold on 

 the earth is inevitable. The struggle would be more intense 

 between those organisms which demand the same kind of food, 

 that is, among kindred. This is the fundamental struggle. It 

 would be complicated by the fact that some groups of animals 

 prey upon others, and that the primary conditions of life, as 

 water, temperature, etc., are subject to striking changes. These 

 facts tend, by just so much as they destroy individuals, to 

 relieve the struggle within the species, and to introduce new fac- 

 tors which give great variety and interest to the life problems of 

 animals. There is nothing more certain than that this struggle 

 has occupied organisms practically from the beginning, and all 

 our explanations of present conditions must take note of the 

 fact. All the important structures and activities of animals are 

 modified by this competition for a livelihood. 



136. Library Exercises. The student should be invited to make real to him- 

 self the possibilities of a geometrical increase as applied to organisms. Take 



