The iNTERPRETATrON OF STRUCTURE. II 



basis, because the factors, cgritrolling thejn 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 tpye 

 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. A recognized principle of 

 ertibryology 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 com-- 

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

 presum.ably its own ancestral, forms. That of com.parative 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-fgatures a low gjTada 

 of structure of a kind possessed by the ancestors of existing higher, 

 forms. These relations form a basis for the'j^omparison of; the 

 em.bryonic development of organisrhs with tl^,-|^olution or history, 

 pf the groups which they represent, the forn^r being distinguished 

 as ontogeny, the latter as phylogeny. The interpretatiop^fjthe 

 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 guch 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 

 of development of thfe 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 succesgion of generations, or continuity 

 of life, carries onward tjie stfucture of the body, and as fossil 

 (?rganisms reveal, has maintained, this process for countless millions 

 of years. 



