96 ZOOLOGY. 



directly. Only a limited amount of vegetation can. be sup- 

 ported by the earth without cultivation. The number of ani- 

 mals 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 life time 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 

 factors which give great variety and interest to the life prob- 

 lems of animals. There is nothing more certain than that this 

 struggle has occupied organisms practically from the begin- 

 ning, 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. 



132. Library Exercises. The student should be invited to make real 

 to himself the possibilities of a geometrical increase as applied to organ- 

 isms. Take the known rate of increase (that is, the total number of 

 descendants in an average lifetime) of a number of common animals and 

 determine the possible living descendants in a .specified time. Find ref- 

 erences concerning infusoria, insects, fish, man. Have you any observa- 

 tions relating to the reality of the struggle for food among animals? 



