INDIVIDUAL DIFFERENTIATION AND ADAPTATION. 131 



1 68. Summary. 



1. It is necessary to consider the individual not merely as 

 a group of cells and tissues but as a unit acting and being acted 

 upon by all external forces and by other organisms. 



2. Characteristics derived from the parents through the 

 union of the sexual cells, or by the non-sexual modes of repro- 

 duction, are described as hereditary. Other parental influences 

 have nothing to do with heredity. Heredity may preserve old 

 qualities or give rise to new ones. 



3. Individuals vary as the result (i) of internal conditions 

 and changes, the causes of which are obscure, and (2) of dif- 

 ferences in the environment. The environment may produce 

 very important changes during the single life of the individual. 



4. The food supply of animals is limited, since all ultimately 

 depend on plants; any species multiplying at its average rate 

 of increase could in a short time, if unchecked, stock the earth 

 up to its limits of support ; that this does not occur is due to a 

 struggle for food among the excessive numbers which are 

 born, whereby only a small percentage of them reach maturity. 

 In the main, those survive which possess some qualities which 

 tend to fit them for the environment in which they find them- 

 selves. These are thus enabled to transmit their qualities to 

 their offspring, the fittest of which are again chosen. The 

 result is adaptation, and the process is known as natural 

 selection. 



5. A similar result is effected by man in domestic animals 

 by artificially selecting individuals in accordance with the pos- 

 session of certain features. The resulting forms are frequently 

 very unsuited to the natural environment, and could not sur- 

 vive if left to themselves. 



6. As the result of various causes animals become dispersed 

 from their point of origin, and in becoming adapted to the 

 different regions into which they go, or through variation 

 within a given region, give rise to new varieties. When, by 

 any means, these groups have become perfectly adapted to 

 their new special environment and permanently different from 



