40 OKNEHAL PIUNCIl'LKS OF ZOOLOGY. 



completely ossified vertebral column; their embryos, on the other 

 hand, have in the early stages only the notoehord (amphioxns 

 stage); later this notocliord becomes constricted by the vertebra^ 

 (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) notoehord, (2) 

 notoehord 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 tliose of embryology. But in reality "\ve 

 should exjoect a threefohl parallelism: for according to the theory 

 of evolution the systematic arrangement of animals is conditioned 

 Ijy a third factor — the historical development of the animal world, 

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

 give tlie 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, A); that from this the 

 heterocercal (/>'), and from the heterocercal the homocercal form 

 of tail-fin (0, I)) can be derived. Embryologically the most liighly 

 develoj)ed fishes are first dii^hycercal, later heterocercal, and 

 finally become homocercal. Last of all, paleontologically the 

 oldest fishes are diphycercal or heterocercal, and only later do 

 liomocercal forms a])pear. 



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 morphologv 

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

 tion. 



Distribution of Animals. — From Animal (Jeography we learn 

 that the present distribution of animals is the 2)roduct of past 

 luindreds and thousands of years. It will therefore be possible 

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

 l)y proceeding with the utmost caution and overcoming extreme 

 difficulties. 



Jf we assume that from the beginning all animal species were 

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

 the purposeful (Ireator in the regions best suited to their organiza- 

 tion; their distribution would therefore have been determined bv 



