The Interpretation of Structure. ii 



basis, because the factors controlling' them lie outside of the body 

 of the individual, and are such as have operated, only through a long 

 series of gradually changing conditions in the evolution of its type. 

 As applied to a particular animal, the morphological method, con- 

 sists in explaining its adult structure by reference either to its 

 embryonic development, or to the equivalent conditions in lower 

 existing, or perhaps fossil, forms. The recognized principle of 

 embryology is that known as the Law of Recapitulation. It is 

 based on the general observation that the definitive structure of ani 

 organism is attained, through a series of embryonic stages, in which j 

 it not only develops from a simple or ground type to a more com- 

 plex condition but also reflects in passing the features of lower, and 

 presumably its own ancestral, forms. That 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 low grade 

 of structure of a kind 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 is a matter of distinguishing its 

 more general features from its more special ones, the former being 

 in all cases those to which the ontogenetic and phylogenetic prin- 

 ciples are especially applicable. 



How such conditions, affecting the present form of an organism, 

 have come about, may be explained, by reference to ancestry. .The 

 sum of characters, apart from influences of accident, are the result 

 ot development of the primordial cell which constitutes the fertilized 

 egg. Such features as are impressed, upon the animal during 

 growth or maturity are in this respect negligible, and the importance 

 of the egg-cell is in no way diminished by the fact that in the 

 majority of mammals it undergoes its early development within 

 the maternal body. The succession of generations, or continuity 

 of life, carries onward the structure of the body, and as fossil 

 organisms reveal, has maintained this process for countless millions 

 of years. 



