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390 DENTAL ANATOMY.

be accompanied with an increased secretion and great distension of the
poison-glands.
In reference to the poisonous character of this secretion, it is a well-
known fact that the normal saliva of many animals is more or less
dangerous when injected directly into the blood, and that in a state of
rage it is rendered more so. Prof. Cope has recently called my atten-
tion to the possible explanation of the poisonous character of this anal-
ogous secretion of the venomous serpents : that since their peculiar
method of locomotion would expose them most frequently to injuries
and inconveniences calculated to excite this state, the normal salivary
secretions have become accordingly modified.
The remaining orders of the Reptilia do not exhibit any important
modifications of the dental system worthy of special notice.

THE TEETH OF THE MAMMALIA.

With a consideration of the teeth of the Manmialia we enter upon a
study of a series of dental organs whose complexity, variety, and s])e-
cialization surpass those of any other group of the Vertebrata. The
wide diversity of conditions under which the different members of this
great group exist would of itself lead one to anticipate a corresponding
diversity in dietetic habits, as well as organs suitable for the prehension
and assimilation of the substances by which they are nourished. The
broad grinding surface afforded by the molar tooth of the elej)hant, the
sharp, trenchant, sectorial dentition of the lion, the great scalj^riform
incisors of the beaver, the small cylindrical teeth of the armadillo, are
a few examples of the great range of variety which mammals exhibit
in the form of their dental organs.
As already remarked in the introductory pages, this study is greatly
facilitated by considering it from the standpoint of evolution, or rather
in the light of the palfeontological history of the group. If Me look
upon the fossil remains of any given period of geologic time as the
representatives in part of the animals which at that time inhabited the
earth, it then becomes of the utmost importance to ascertain the exact
relationship Avhich the animals of each period bear to those which have
preceded and succeeded them in time. It is needless to say that the
conclusions which we are com])elled to draw from studies of this cha-
racter are im])ortant and significant, and serve to bring into the closest
connection many isolated facts which if considered by themselves would
be wholly unintelligible.
Some objection to this method of treatment will doubtless be raised
by those who do not accept evolution as a demonstrated fact, or those,
again, Avho consider our information concerning extinct forms too meagre
for purposes of generalization. In answer to these objections it must be
urged that jxdreontological law com]>els us to recognize the important fact
that in every department of life the generalized has preceded the spe-