Page 491 - My FlipBook

TEETH OF THE VERTEBRATA. 501

CONCLUSIONS, ACKNOWLEDGMENTS, ETC.
Throughout the foregoing pages I have endeavored not only to
give the leading eharaeteristics of the principal niodiiieations of the
dental organs of the A'ertebrata, bnt have in many oases, so far as our
knowledge of the extinct forms would permit, endeavored to trace the
leading steps in the production of the complex from the simple form.
In so doing I have been made aware of the difficulties which beset such
an undertaking: the principal burden of these difficulties lies in the com-
paratively imperfect knowledge we possess of the palffiontological history
of certain groups. In others the ancestry is more clearly indicated, and in
my judgment the evidence is sufficient to demonstrate with a reasonable
degree of certainty the more important steps in their dental evolution.
The modification of an organ from a simple to a complex structure
necessarily implies a cause or force, adequate to the production of such
result. What, then, is the nature of the force or forces involved, and
what is their method of operation ? To simply say that this or that is
so, that this tooth is simple and that is complex, without giving any
reason why it is so, conveys little information. If one tooth is sim-
ple and another complex, there are reasons for it, and it is not only
within the province, but is clearly the duty, of the odontologist to dis-
cover and p(jint out these reasons if they can be found to exist.
Two explanations for all such phenomena have been offered. One
of these presumes that they were created so by supernatural forces, but
as to the nature of these forces we are not informed ; much less do we
know about the manner in which it was done. The other assumes that
the natural or physical forces, operating through distinct and well-known
physiological laws, are alone responsible for the resulting modifications.
Between these tAvo explanations the naturalist experiences little diffi-
culty in deciding which is most in accordance with the observed facts
at his command. While the one rests solely upon the vaguest assump-
tion, unsupported by so much as a single fact, the other rests upon
observed scientific truth, which any one can verify who will take the
pains to investigate. When we ascribe these modifications to the physi-
cal forces, the conclusion seems inevitable that those of a mechanical
nature have been most largely concerned in the modification of form.
The change in form or size of any organ is principally due to addi-
tion, subtraction, or transposition of the histological elements of which
it is composed ; these, as is well knoAvn, are directly dependent on the
amount of physiological waste and repair which the organ sustains, or,
in other words, the extent of use and disuse. In pro])ortion as an organ
or a part of an organ is used, in that proportion will there be increased
destruction of its substance and a corresponding determination of the
nutritive fluids to supply the loss. The reverse is true of disuse.
In the harder tissues of the animal body strain and pressure have
likewise been potent factors in the determination of form. Recognizing
the importance of these influences, Mr. J. A. Ryder has constructed a
most ingenious and far-reaching hypothesis in regard to the teeth, which
he terms ''the mechanical genesis of tooth-forms."' In this he satis-
' Proceedings Acad. Nat. Sciences, Philada., 1878.