THE PATH OF DEGENEKATION IN ANIMALS 179 



Using the method of embryology and the prin- 

 ciple, so far as it can be followed, that ontogeny 

 recapitulates phylogeny, we shall investigate the 

 mode of origin of some reduced structures. We 

 shall thus learn if the organs in degenerating 

 resume any of their ancestral stages. 



Reduced organs may appear in two different 

 forms. They may be atrophied after having 

 reached a more or less complete development, 

 and in this case we shall have to compare the 

 course of the atrophy with the course of the 

 development. They may be rudimentary, that 

 is to say. their development may have been 

 arrested at a given point, the adult state never 

 being attained. In this case, .so far as ontogeny 

 repeats phylogeny, the arrest of these organs at 

 different stages in different species should furnish 

 a series with greater or smaller lacunae, but a 

 series which will be the reversed series of the 

 original stages in phylogeny. 



SECTION I. 

 The path of degeneration in animals. 



1. Morphology and Embryology. The law of re- 

 capitulation. It has been so often repeated that the 

 individual development of an organ is a resume more 

 or less compressed of its historical evolution, that 

 people are apt to attempt too exact an application 



