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TEETH OF THE VERTEBRATA. 391

cializeJ in time ; we pass from the simple to the complex, whether an
individual organ or the entire organism be considered ; and the teeth
form no exception to this rule. So conclusive is the testimony which
it is now possible to adduce in support of this general proposition, and
so pregnant are the minds of modern biologists with this belief, that it
seems utterly impossible to escape the conviction that life from its earli-
est inception has been continuously, and in many instances progressively,
modified. As to the nature of the causes which have induced this modi-
fication, there is much less unanimity of opinion. It is a question
regarding which the most exhaustive philosophic discussion is now in
progress.
When we speak of the origin of mammalian teeth, it is necessary to
have some definite knowledge of the origin of this class of animals before
we can be absolutely certain of just what constitutes a primitive mamma-
lian dentition. Unfortunately, the evidence which would enable us to
determine the ancestry of the mammal beyond dispute has not as yet
been found, but it appears sufficiently evident that we are limited in our
choice to the Batrachia and Reptilia of the Permian Period. Huxley,
who has devoted considerable attention to this subject, concludes that
we must go backward past the Reptilia directly to the Batrachia. This
conclusion is primarily based upon a comparison of the pelvic arch of
the monotremes with that of the batrachians. In addition to the evi-
dence drawn from this source, upon which his argument is principally
founded, the following reasons are given for this view : " The Batrachia
are the only air-breathing Vertebrata which, like the Mammalia, have a
dicondylian skull. It is only in them that the articular elements of the
mandibular arch remain cartiUiginous, while the quadrate articulation
remains small, and the squamosal extends down over the osseous ele-
ments of tlie mandible, thus affording an easy transition to the mam-
malian condition of those parts. Tlie pectoral arch of the monotremes
is as much batrachian as it is reptilian or avian. The carpus and tarsus
of all Reptilia and Aves, except the turtles, are modified away from the
batrachian tyjjc, while those of the mammal are directly reducible to it.
Finally, the fact that in all Reptilia and Aves it is a right aortic arch
which is the main conduit of arterial blood leaving the heart, while in
the Mammalia it is the left which performs this office, is a great stum-
bling-block in the way of the derivation of the INIanunalia from any of
the Reptilia or Aves. But if we suppose the earliest forms of both
Reptilia and jNIammalia to have had a common batrachian origin,
then there is no difficulty in the supposition that from the first it Avas
the left aortic arch in the one series, and the right aortic arch in the
other, which became the predominant feeder of the arterial system."
If we had only the recent forms to consider, the argument advanced
by this learned anatomist would be specially potent ; but when we study
carefully the osteology of the Reptilia of the Permian Period, many of
the arguments here advanced are invalidated. The structure of the
pectoral and pelvic arches of the theromorph Reptilia, as has been ascer-
tained by Cope, resembles that of the monotremes far more than does
that of any kno\vn batrachian. The carpus and tarsus of these forms
are almost identical with those of the monotremes, Avhile comparatively