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 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 
embryology is that known as ihe 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 
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 
of 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. 
