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 zoological 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 



