DIFFERENTIATION OF INDIVIDUALS AND ADAPTATION lOS 



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 power of reproduction in a geometric ratio, 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 conditions tend, by just so much as they de- 

 stroy individuals, to relieve the struggle within the species. 

 The interaction of these processes gives great variety to the life 

 problems of animals and great interest to their study. There 

 is nothing more certain than that this struggle has occupied 

 organisms practically from the beginning, and all our explana- 

 tions 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 is invited to make real to himself the 

 possibilities of a geometrical increase as applied to organisms. 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 references concerning infusoria, insects, fish, man. 

 Have you any observations relating to the reaUty of the struggle for food among 

 animals? 



137. Natural Selection. — In spite of this power of reproduc- 

 tion we see that, on the average, the mass of individuals does 

 not increase. The earth is no more thickly inhabited by ani- 

 mals today than it has been for countless ages. The propor- 

 tions of different animals vary now and again, but that is all. 

 Out of a family of one hundred young individuals striving for a 

 foothold, no two of which are alike, ninety-eight on the average 

 will be destroyed in a species which is just holding its own. 

 Which will survive? Barring accidents, specially favoring or 

 beyond the powers of any of the individuals to resist, those will 



