INTERPRETATION OF STRUCTURE 3 



development or to the equivalent conditions in lower forms, 

 existing or fossil. A recognized principle of embryology is that 

 known as the Law of Recapitulation. It is based on the general 

 observation that the definitive structure of an organism is attained 

 through a series of embryonic stages, in which it not only develops 

 from a simple or ground type to a more complex condition but 

 also reflects in passing the features of lower forms which presumably 

 represent its ancestors. These features are mingled with, however, 

 and sometimes obscured by newer, purely embryonic characteristics 

 which are not ancestral. Moreover, the adult condition of an ad- 

 vanced animal is sometimes reached by the retention of a character- 

 istic which was only embryonic in its predecessors. The application 

 of comparative anatomy depends on the comparison of higher, 

 specialized animals with lower, or generalized ones, the latter being 

 assumed, in one feature or another, to have remained in a backward 

 or primitive state of specialization, and therefore to reflect in such 

 features a grade of structure comparable with that possessed by 

 the ancestors of existing higher forms. These relations form a 

 basis for the comparison of the embryonic development of organisms 

 with the evolution or history of the groups which they represent, the 

 former being distinguished as ontogeny, the latter as phylogeny. 

 The interpretation of the adult structure of an organism involves 

 the distinction of its more general features from the more special 

 ones and the application to them of ontogenetic and phylogenetic 

 principles. 



The present form common to the individuals of one kind of 

 animal may be explained only by reference to ancestry. Apart 

 from influences of accident, the sum of characters of the individual 

 is the result of development, under more or less fixed environmental 

 conditions, of the primordial cell which constitutes the fertilized 

 egg. So long as the environment remains comparable with those nor- 

 mal for the species, such features as are impressed upon the animal 

 during growth or maturity are negligible in this connection, the 

 developmental possibilities of the- fertilized egg having been trans- 

 mitted to it through the succession of generations. Through this 

 succession the continuity of life, as the fossil remains of organisms 

 of the past reveal, has carried onward the structure of the body for 

 countless millions of years. 



