194 th:e history of creation. 



Here, as everywhere else, analogies must be traced to Adapta- 

 tion, homologies to Transmission by Inheritance. "When we 

 see that the limbs of the most different Vertebrata, in spite 

 of their exceedingly different external forms, nevertheless 

 possess essentially the same internal structure ; when we see 

 that in the arm of a man and ape, in the wing of a man or 

 a bird, in the breast fins of whales and sea-dragons, in the 

 fore-legs of hoofed animals and frogs, the same bones 

 always lie in the same characteristic position, articulation 

 and connection — we can only explain this wonderful agree- 

 ment and homology by the supposition of a common trans- 

 mission by inheritance from a single primary form. On 

 the other hand, the striking differences of these homologous 

 bodily parts proceed from adaptation to different conditions 

 of existence. (Compare Plate IV.) 



Ontogeny, or the individual history of development, like 

 comparative anatomy, is of especial importance to the pedi- 

 gree of the Vertebrata. The first stages of development 

 arising out of the egg are essentially identical in aU 

 Vertebrate animals, and retain their agreement the longer, 

 the nearer the respective Vertebrate animal forms, when 

 fuUy developed, stand to one another in the natural system, 

 that is, in the pedigree. How far this agreement of germ 

 forms, or embryos, extends, even in the most highly developed 

 Vertebrate animals, I have already had occasion to explain 

 (vol. i. pp. 306-309). The complete agreement in form 

 and structure, for example, in the embryos of a man and 

 a dog, of a bird and a tortoise, existing in the stages of 

 development represented on Plates II. and III, is a fact 

 of incalculable importance, and furnishes us with the most 

 important data for the construction of their pedigree. 



