METAMORPHOSIS. 213 



embryology of the different groups of the animal 

 kingdom reveals the fact that in no case does the 

 development proceed with equal rapidity at all points 

 of its course from inception to maturity. As before 

 remarked, toward the end of its course, development 

 becomes slower and less marked in its changes. As 

 pointed out, this is due to lack of strength in the 

 association of the hereditary impulses, and conse- 

 quent lack of progressive change in the sum and 

 relation of the internal and external forces. This 

 progressive change in the relation between the in- 

 ternal and external forces is the cause of develop- 

 ment. The progressive change is most constant 

 and invariable for long successions of generations, 

 in the earliest stages of development before the 

 animal leaves the egg, or the body of the parent, 

 and therefore at this period the development is most 

 rapid and uniform. When the animal leaves this 

 earliest protection, and goes forth a very imperfectly 

 developed creature to shift for itself, it then meets 

 with conditions that are slightly different for each 

 generation and each individual. At this period of 

 its life, therefore, because of its variable relations 

 to variable conditions, the associated reactions of the 

 hereditary impulse will not be so strong, and devel- 

 opment will accordingly be slower and less uniform. 

 For it is a general fact, that those associations are 



