ternal condition, power, or force
which must be conceived as occasioning such ready modifiability. But indeed
it is not at all easy to see how the influence of the surface of the ground
or any conceivable condition or force can produce the difference which
exists between the ventral and dorsal shields of the carapace of a
tortoise, or by what differences of merely external causes the ovaries of
the two sides of the body can be made equal in a bat and unequal in a bird.

[Illustration: AN ECHINUS, OR SEA-URCHIN.
(The spines removed from one-half.)]

There is, on the other hand, an _a priori_ reason why we should expect to
find that the symmetrical forms of all animals are due to internal causes.
This reason is the fact that the symmetrical forms of minerals are
undoubtedly due to such causes. It is unnecessary here to do more than
allude to the beautiful and complex forms presented by inorganic
structures. With regard to organisms, however, the wonderful Acanthometrae
and the Polycystina may be mentioned as presenting complexities of form
which can hardly be thought to be due to other than _internal_ causes. The
same may be said of the great group of Echinoderms, with their amazing{168}
variety of component parts. If then internal forces can so build up the
most varied structures, they are surely capable of producing the serial,
lateral, and vertical symmetries which higher animal forms exhibit. Mr.
Spencer is the more bound to admit this, inasmuch as in his doctrine of
"physiological units" he maintains that these organic atoms of his have an
innate power of building up and evolving the whole and perfect animal from
which they were in each case derived. To build up and evolve the various
symmetries here spoken of is not one whit more mysterious. Directly to
refute Mr. Spencer's assertion, however, would require the bringing forward
of examples of organisms which are ill-adapted to their positions, and out
of harmony with their surroundings--a difficult task indeed.[168]

Secondly, as regards the last-mentioned author's explanation of such serial
homology as exists in the centipede and its allies, the very groundwork is
open to objection. Multiplication by spontaneous fission seems from some
recent researches to be much less frequent than has been supposed, and more
evidence is required as to the fact of the habitual propagation of _any_
planariae in this fashion.[169] But even if this were as asserted, {169}
nevert