194 THE 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 all 
Vertebrate animals, and retain their agreement the longer, 
the nearer the respective Vertebrate animal forms, when 
fully 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 Il. and III, is a fact 
of incalculable importance, and furnishes us with the most 
important data for the construction of their pedigree, 
