INDIVIDUAL DIFFERENTIATION AND ADAPTATION. 99 



centre of multiplication under the influence of hunger or un- 

 favorable local conditions. They are often assisted in these 

 dispersals by such natural agencies as winds, currents of water, 

 and by other animals. If the migrating forms succeed in find- 

 ing new regions suited to their needs, there results a condition 

 of adaptation between organisms and their respective environ- 

 ments, but without any active change in the characteristics of 

 the organism. The environment itself is subject to continual 

 change and there are too many barriers in the way of universal 

 migration for this to be accepted as a complete explanation of 

 the widely observed adjustment of animals to the conditions 

 which surround them. 



In the second place animals may become suited to their en- 

 vironment by variation, without migration. There is no ques- 

 tion that this also occurs and that it is the more important 

 factor of the two. It has been shown (129) that all animals 

 are variable. Students of biology have suggested two impor- 

 tant ways in which variations may give rise to a harmony 

 between the organism and its surroundings. This result may 

 take place through natural selection (133). According to this 

 view the organisms naturally tend to vary. The changing en- 

 vironment stimulates this tendency to variation. Out of a 

 thousand individuals of similar parentage there will be numer- 

 ous slight differences of structure and physiological qualities. 

 Some of these will be more, and some less, favorable to the 

 environment. In the struggle those will be eliminated which 

 for any reason are strikingly unsuited to the environment. On 

 the other hand those animals whose variations are most in 

 accordance with the local condition will persist and propagate 

 their kind, tending through heredity to pass on to their off- 

 spring the qualities which enabled them to adjust themselves 

 to their surroundings. Thus there will be a gradual, ever 

 increasing adaptation in the whole species of which they are a 

 part, by natural selection. Occasionally there occurs in off- 

 spring, from the action of the environment or from other 

 causes, a sudden and considerable change from the parent 



