olumn, are explained by Mr. Spencer as the results of alternate strains
and compressions acting on a primitively homogeneous cylinder. The serial
homology of the fore and hind limbs is explained by the same writer as the
result of a similarity in the influences and conditions to which they are
exposed. Serial homologues so formed might be called, as Mr. Ray Lankester
has proposed, "homoplastic." But there are, it is here contended, abundant
reasons for thinking that the predominant agent in the production of the
homologies of the limbs is an _internal_ force or tendency. And if such a
power can be shown to be necessary in this instance, it may also be
legitimately used to explain such serial homologies as those of the
centipede's segments and of the joints of the backbone. At the same time it
is not, of course, pretended that external conditions do not contribute
their own effects in addition. The presence of this internal power will be
rendered more probable if valid arguments can be brought forward against
the explanations which Mr. Herbert Spencer has offered.

_Lateral homology_ (or bilateral symmetry) is the resemblance between the
right and left sides of an animal, or of part of an animal; as, _e.g._,
between our right hand and our left. It exists more or less at one or other
time of life in all animals, except some very lowly organized creatures. In
the highest animals this symmetry is laid down at the very dawn of life,
the first trace of the future creature being a longitudinal streak--the
embryonic "primitive groove." This kind of homology is explained by Mr.
Spencer as the result of the similar way in which conditions affect {165}
the right and left sides respectively.

[Illustration: VERTEBRAE OF AXOLOTL.]

_Vertical homology_ (or vertical symmetry) is the resemblance existing
between parts which are placed one above the other beneath. It is much less
general and marked than serial, or lateral homology. Nevertheless, it is
plainly to be seen in the tail region of most fishes, and in the
far-extending dorsal (back) and ventral (belly) fins of such kinds as the
sole and the flounder.

It is also strikingly shown in the bones of the tail of certain efts, as in
_Chioglossa_, where the complexity of the upper (neural) arch is closely
repeated by the inferior one. Again, in _Spelerpes rubra_, where almost
vertically ascending articular processes above are repeated by almost
vertically descending articular proc