esses 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. What is there here either in the darkness, or the friction, or in any
other conceivable external cause, to have produced the first beginning of
such an unprecedented displacement of the eye? Mr. Spencer has beautifully
illustrated that correlation which all must admit to exist between the
forms of organisms and their surrounding external conditions, but by no
means proved that the latter are _the cause_ of the former.

[Illustration: PLEURONECTIDAE, WITH THE PECULIARLY PLACED EYE IN DIFFERENT
POSITIONS.]

Some internal conditions (or in ordinary language some internal power and
force) must be conceded to living organisms, otherwise incident forces must
act upon them and upon non-living aggregations of matter in the same way
and with similar effects.

If the mere presence of these incident forces produces so ready a response
in animals and plants, it must be that there are, in their case, conditions
disposing and enabling them so to respond, according to the old maxim,
_Quicquid recipitur, recipitur ad modum recipientis_, as the same rays of
light which bleach a piece of silk, blacken nitrate of silver. If,
therefore, we attribute the forms of organisms to the action of {167}
external conditions, _i.e._ of incident forces on their modifiable
structure, we give but a partial account of the matter, removing a step
back, as it were, the action of the in