Again, in _Spelerpes rubra_, where almost vertically ascending articular processes above are repeated by almost vertically descending articular processes below.

Also in the axolotl, where there are douple pits, placed side by side, not only superiorly but at the same time inferiorly.[167] This kind of homology is also explained by Mr.Spencer as the result of the similarity of conditions affecting the two parts.

Thus he explains the very general absence of symmetry between the dorsal and ventral surfaces of animals by the different conditions to which these two surfaces are respectively exposed, and in the same way he explains the asymmetry of the flat-fishes (_Pleuronectidae_), of snails, &c.
Now, first, as regards Mr.Spencer's explanation of animal forms by means of the influence of external conditions, the following observations may be made.

Abundant instances are brought forward by him of admirable adaptation of structure to circumstances, but as to the immense majority of these it is very difficult, if not impossible, to see _how_ external conditions{166} can have produced, or even tended to have produced them.

For example, we may take the migration of one eye of the sole to the other side of its head.