4 THE DEVELOPMENT OF THE CHICK 
ticulars, conditions that are adult in lower species of the same 
phylum; and, moreover, the order of embryonic development 
of organs corresponds in general to the taxonomic order of organ- 
ization of the same organs. As the taxonomic order is the order 
of evolution, Haeckel’s generalization, which he called the funda- 
mental law of biogenesis, would appear to follow of necessity. 
But it never happens that the embryo of any definite species 
resembles in its entirety the adult of a lower species, nor even 
the embryo of a lower species; its organization is specific at all 
stages from the ovum on, so that it is possible without any diffi- 
culty to recognize the order of animals to which a given embryo 
belongs, and more careful examination will usually enable one 
to assign its zodlogical position very closely. 
If phylogeny be understood to be the succession of adult 
forms in the line of evolution, it cannot be said in any real sense 
that ontogeny is a brief recapitulation of phylogeny, for the 
embryo of a higher form is never like the adult of a lower form, 
though the anatomy of embryonic organs of higher species re- 
sembles in many particulars the anatomy of the homologous 
organs of the adult of the lower species. However, if we conceive 
that the whole life history is necessary for the definition of a 
species, we obtain a different basis for the recapitulation theory. 
The comparable units are then entire ontogenies, and these re- 
semble one another in proportion to the nearness of relationship, 
just as the definitive structures do. The ontogeny is inherited 
no less than the adult characteristics, and is subject to precisely 
the same laws of modification and variation. Thus in nearly 
related species the ontogenies are very similar; in more distantly 
related species there is less resemblance, and in species from 
different classes the ontogenies are widely divergent in many 
respects. 
In species of lower grades of organization the ontogenetic 
series is a shorter one than in species of higher grades, so that 
the final stages of the organs of a lower species become inter- 
mediate or embryonic stages in species of higher rank. But the 
stage of the lower species does not appear in all the organs of the 
higher species simultaneously. Thus the chick never exhibits 
the grade of organization of a fish throughout; while its pharynx, 
for instance, is in a fish-like condition with reference to arches 
and clefts, the nervous system is relatively undifferentiated, and 
