PART III. 



MECHAIs^IOAL EYOLUTIOE". 



XII. 



THE EELATION OF ANIMAL MOTION TO ANIMAL 



EVOLUTION. 



To explain the origin of variation in animal structure is, par 

 excellence, the object of the doctrine of evolution. There can be 

 little doubt that the law of natural selection includes the cause of 

 the preservation of certain modifications of pre-existent structure, 

 in preference to others, after they have been brought into exist- 

 ence. In what manner or by what process the growing tissues of 

 young animals have been so affected as to produce some organ or 

 part of an organ which the parent did or does not possess, must 

 be explained by a different set of laws. These have been termed 

 originative^ while those involved in natural selection are restrictive 

 only. 



I. 



Of course we naturally look to something in the *^ surrounding 

 circumstances" in which a plant or animal is placed, or its "en- 

 vironment," as the most probable stimulant of change of its char- 

 acter, because we know that such beings are totally dependent on 

 cosmic and terrestrial forces for their sustenance and preservation. 

 The difficulty has been to connect these forces with change of 

 structure SLS originative ; to show their operation as multiplying, 

 restricting or destroying organisms already in existence is compar- 

 atively easy. This difficulty is partially due to the fact that such 

 modifications must be realized during a limited portion of the life 

 of an animal at least ; that is, during the period of growth, when 

 it is not at all or but little subject to the influence of external en- 

 vironment, but is usually protected or supported by the parent. 



That the environment and changes in it affect the movements 



