INTRODUCTION 19 



Mr. Sedgwick has lately elaborated this last suggestion 

 into the general theory that ancestral characters are only 

 retained in the embryo when the ancestral condition was 

 once a larval condition, which has more recently become 

 embryonic, in consequence of the retention of the larva in 

 the egg, or within the body of the mother, until after its 

 metamorphosis. 



There are numerous instances of the truth of Mr. Sedg- 

 wick's theory, but it is not a general theory of individual 

 development. The general truth is that when the habits 

 and conditions are different at different periods of the 

 individual life, then and to a proportionate degree will the 

 structure of the individual be different during those periods. 

 It may be said that this is merely another way of stating 

 the principle of adaptation as the result of natural selection, 

 the most advantageous variations being selected at each 

 stage of life separately. But my contention is that we have 

 no evidence that the necessary variations ever occurred until 

 the particular habits and conditions produced them. If it 

 is difficult to believe that the modifications were independent 

 of external conditions, when we compare different adapta- 

 tions in different individuals, it is still more difficult when 

 we compare successive stages of the same individual, and 

 follow in actual progress the changes of structure and function 

 corresponding with change of habits and conditions. To my 

 mind the phenomena of metamorphosis can only be explained 

 as due to the heredity of modifications of growth originally 

 caused directly in the individual by changed conditions 

 which introduced new irritations, new stimulations, and 

 changes of function. 



This matter again can only be studied in particular in- 

 stances. The most familiar case is that of the frog and other 

 amphibia. We can have no doubt that the air-breathing 

 amphibia were evolved from fishes, though we may not be able 



