I08 ZOOLOGY 



variation, without migration. There is no question that this 

 also occurs, and it may be 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 inherited 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 inherited variations 

 are most in accordance with the local condition will persist and 

 propagate their kind, tending through heredity to pass on to 

 their offspring the native 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 

 quaHties are transmitted, and the fluctuations of body pro- 

 duced by use and environment cannot be transmitted, natural 

 selection could not act, by way of the acquired characters but 

 only through happy mutations, in improving a species. 



Occasionally there occurs in offspring, whether from the 

 action of the environment upon the germ plasm or from other 

 inner causes, a sudden and considerable change from the parent 

 type. Such a mutation product is known as a "sport." It is 

 quite possible that natural selection may seize on such mutations 

 and, if in a favorable direction, preserve and increase them. 

 In such cases adaptation might take place with great rapidity, 

 instead of in the gradual way described above. 



In the second place it has been argued that fortunate muta- 

 tions are not just happy coincidences, but that organisms are 

 so attuned to conditions that they naturally tend to make 

 definite and suitable variations. In other words this view holds 

 that the majority of the variations brought about by a given 



