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. 
Phylogeny, 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 Archxopteryx 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. 
Ontogeny, 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 
