ANATOMY 13 



and Reptiles, and by concluding that the avine ankle-joint is 

 produced by the fusion of the proximal tarsal bones with the 

 tibia, and of the distal tarsals with the metatarsals, that conse- 

 quently this joint in Birds is not the same as the ankle-joint 

 of Mammals. If moreover, as is the case here, the study of the 

 embryonic development of Birds shews that this fusion actually 

 does take place, Ontogeny corroborates the correctness of the 

 conclusions which we had arrived at by the strictly comparative or 

 phylogenetic method. 



Fhylogeny, then, is the study of the relationship and the 

 descent of the various animals, often with the help of fossil species, 

 which are generally in some ways intermediate between other recent 

 forms. For instance, through comparison of the skeleton of Birds 

 with that of other Vertebrates, we find that Birds resemble Rep- 

 tiles much more than they do Fishes or Amphibia or Mammals ; 

 this we express by saying that Birds are rather nearly related to 

 Reptiles ; the extraordinary resemblance of recent Birds with the 

 fossil Archxopteryx, which at the same time has still many truly 

 Reptilian characters, links the two classes still more together. We 

 conclude that Reptiles and Birds are descendants of one common 

 Reptilian stock. Since most Reptiles possess teeth, and the more 

 than half avine Archxoptenjx also has teeth, we again conclude 

 that the earliest Birds likewise possessed such organs, and that 

 their descendants have lost them. In this belief we are not shaken, 

 although the most careful examination of embryonic birds has 

 failed to reveal even the smallest traces of dental germs. The 

 subsequent discovery in American cretaceous deposits of Toothed 

 birds, like Enaliornis and Hesperornis, is a beautiful corroboration of 

 the soundness of the method. 



Ontogmy, on the other hand, includes the study of the develop- 

 ment of the individual, and hence is often called Embryology. What- 

 ever organic modifications the parents have acquired during their 

 life, subjected to the struggle for existence, be it through natural or 

 sexual selection, or be it through spontaneous variation, will be 

 inherited, at least partly, by their offspring. Ontogeny is therefore 

 the recapitulation by the growing individual of the sum total of the 

 ever-changing stages and conditions through which the whole chain 

 of its ancestors has passed : it is a condensed repetition of Phylo- 

 geny. This repetition is often so much condensed that many 

 previous stages are rapidly passed through, or may even be appar- 

 ently left out, or they have become modified beyond recognition 

 through the development of organs necessitated by, and restricted 

 to, the embryonic stages. Such strictly embryonic organs (for 

 instance the Amnion and the Allantois, or the placenta) are 

 features which have originally nothing whatever to do with the 

 adult, because we know of no Vertebrates which in their adult 



