.e._ by the non-segmentation of the skull. But
in fact the skull _is_ segmented, and, according to the quasi-vertebral
theory of the skull put forward by Professor Huxley,[175] is probably
formed of a number of coalesced segments, of some of which the trabeculae
cranii and the mandibular and hyoidean arches are indications. What is,
perhaps, most remarkable however is, that the segmentation of the
skull--its separation into the three occipital, parietal, and frontal
elements--is most complete and distinct in the highest class, and this can
have nothing, however remotely, to do with the cause suggested by Mr.
Spencer.

Thus, then, there is something to be said in opposition to both the
aggregational and the mechanical explanations of serial homology. The
explanations suggested are very ingenious, yet repose upon a very {173}
small basis of fact. Not but that the process of vertebral segmentation may
have been sometimes assisted by the mechanical action suggested.

It remains now to consider what are the evidences in support of the
existence of an internal power, by the action of which these homological
manifestations are evolved. It is here contended that there _is_ good
evidence of the existence of some such special internal power, and that not
only from facts of comparative anatomy, but also from those of
teratology[176] and pathology. These facts appear to show, not only that
there are homological internal relations, but that they are so strong and
energetic as to re-assert and re-exhibit themselves in creatures which, on
the Darwinian theory, are the descendants of others in which they were much
less marked. They are, in fact, sometimes even more plain and distinct in
animals of the highest types than in inferior forms, and, moreover, this
deep-seated tendency acts even in diseased and abnormal conditions.

Mr. Darwin recognizes[177] these homological relations, and does "not doubt
that they may be mastered more or less completely by Natural Selection." He
does not, however, give any explanation of these phenomena other than the
imposition on them of the name "laws of correlation;" and indeed he says,
"The nature of the bond of correlation is frequently quite obscure." Now,
it is surely more desirable to make use, if possible, of one conception
than to imagine a number of, to all appearance, separate and independent
"laws of correlation" between different parts of each animal.

[Illustration: THE AARD-VARK (ORY