Bird Anatomy

Ontogeny and Phylogeny

Birds Anatomy - Ontogeny and Phylogeny

Descriptive Anatomy in Birds

A word originating from the greek anatome ("dissection"), Anatomy is that branch of zoology which deals with the description of the organic structure of animals.  A branch of this zootomy is Histology, the knowledge of the composition of the tissues of the various organs. The object of Comparative Anatomy is the explanation of the features exhibited by the animal organization. The comparative method examines numbers of different animals (or plants) with reference to the anatomical structure of their various organs, putting similar conditions together, and separating or excluding those which are dissimilar. By observing in such organs their size, number, shape, structure, relative position to other organs, and their development, we ultimately acquire a knowledge of such a series of conditions or features, exhibited by one and the same organ, which in their extremes may appear totally different, but are connected with each other by numerous intermediate stages. By proceeding in such a way, we are, for instance, enabled to understand the ankle-joint of Birds, by comparing the bones of their hind limbs with those of Mammals 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 consequently 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 shows 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,


This 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 Reptiles 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 Archaeopteryx, 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 Archaeopteryx 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.


On the other hand, Ontogeny includes the study of the development of the individual, and hence is often called Embryology. Whatever 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 Phylogeny.  This repetition is often so much condensed that many previous stages are rapidly passed through, or may even be apparently 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 condition live within such bags.

Another imperfection of the ontogenetic record lies in the fact that the sequence in which the various organs are developed in the embryo does not always correspond with the temporary succession in which we know them to have been acquired during the phylogenetic development of the animal in question; thus feathers begin to bud while the skeleton of the embryo is still cartilaginous. Such discrepancies between the ontogenetic and phylogenetic development were termed "caenogenetic" by Professor Haeckel (from the greek kainos meaning "new"). The fact of their frequent occurrence without our being aware of the various cases, warns us to be extremely careful in interpreting the various features exhibited by the embryo. In the present state of our knowledge it is often impossible to decide the taxonomic value of a given feature.

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