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, w^ould 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 w^e 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. 



From this it follows that inheritance of the life-history or 

 ontogeny is the fundamental basis of the recapitulation theory. 

 In the course of evolution terminal or late stages of the life 

 history are modified more rapidly in a visible morphological 

 sense, and earlier stages are more conservative in the same 

 sense. Hence ancestral resemblances adhere incomparably longer 

 to the embryo than to the adult. Ontogenies receive something 

 from every stage of evolution, but they retain most of the 

 previous ontogenetic forms, especially of the early stages, in 



