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.