106 ZOOLOGY 



hunger or unfavorable 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 

 finding new regions suited to their needs, there results a condi- 

 tion of adaptation between organisms and their respective envi- 

 ronments, 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. 



Again animals may become suited to their environment by 

 variation, without migration. There is no question that this 

 also occurs and that it is the more important factor of the two. 

 It has been shown (133) that all animals are variable. Students 

 of biology have suggested two important ways in which varia- 

 tions may give rise to a harmony between the organism and its 

 surroundings. This result may take place through natural 

 selection, which eliminates the unfit (137). 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 en- 

 vironment. 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. If, however, only inherited qualities 

 are transmitted, and the fluctuations of body produced by use 

 and environment cannot be transmitted, natural selection could 

 not act, by way of the acquired characters, in improving a 

 species. 



Occasionally there occurs in offspring, from the action of the 



