40 GENERAL PRINCIPLES OF ZOOLOGY. 



completely ossified vertebral column; their embryos, on the other 

 hand, have in the early stages only the notochord (amphioxus 

 stage) ; later this notochord becomes constricted by the vertebrae 

 (fish-amphibian stage) and finally entirely replaced; the vertebral 

 column is in the beginning cartilaginous, only later becoming ossi- 

 fied. Comparative anatomy and embryology thus give the same 

 developmental stage of the axial skeleton: (1) notochord, (2) 

 notochord and vertebral column, the latter at first formed of 

 cartilage, then of bone. 



We have here spoken of a parallelism between the facts of 

 comparative anatomy and those of embryology. But in reality we 

 should expect a threefold parallelism: for according to the theory 

 of evolution the systematic arrangement of animals is conditioned 

 by a third factor the historical development of the animal world, 

 or phylogeny. The mile-stones of phylogenesis, the fossils, should 

 give the same progressive series in the successive geological strata 

 as the stages of forms found by comparative anatomy and embry- 

 ology. We actually know instances of such threefold parallelisms. 

 Comparative anatomy teaches that the lowest developed form of a 

 fish's tail is the diphycercal (fig. 10, ^4); that from this the 

 heterocercal (B), and from the heterocercal the homocercal form 

 of tail-fin ( C, D) can be derived. Embryologically the most highly 

 developed fishes are first diphycercal, later heterocercal, and 

 finally become homocercal. Last of all, paleontologically the 

 oldest fishes are diphycercal or heterocercal, and only later do 

 homocercal forms appear. 



What has here been referred to is only a small fraction of the 

 weighty proofs which morphology offers in favor of evolution; it 

 can only serve to show how morphological observations can be 

 employed. For the reflecting naturalist the facts of morphology 

 are a single great inductive proof in favor of the theory of evolu- 

 tion. 



Distribution of Animals. From Animal Geography we learn 

 that the present distribution of animals is the product of past 

 hundreds and thousands of years. It will therefore be possible 

 from this to figure out many of the earlier conditions of things, 

 by proceeding with the utmost caution and overcoming extreme 

 difficulties. 



If we assume that from the beginning all animal species were 

 constituted as they now are, they would then have been placed by 

 the purposeful Creator in the regions best suited to their organiza- 

 tion; their distribution would therefore have been determined by 



