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,
Phylogeny
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.
Ontogeny
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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