e
form. Mr. Mivart believes that species change through "an internal force
or tendency," about which it is not pretended that anything is known.
That species have a capacity for change will be admitted by all
evolutionists; but there is no need, as it seems to me, to invoke
any internal force beyond the tendency to ordinary variability, which
through the aid of selection, by man has given rise to many well-adapted
domestic races, and which, through the aid of natural selection, would
equally well give rise by graduated steps to natural races or species.
The final result will generally have been, as already explained, an
advance, but in some few cases a retrogression, in organisation.

Mr. Mivart is further inclined to believe, and some naturalists agree
with him, that new species manifest themselves "with suddenness and by
modifications appearing at once." For instance, he supposes that the
differences between the extinct three-toed Hipparion and the horse arose
suddenly. He thinks it difficult to believe that the wing of a bird "was
developed in any other way than by a comparatively sudden modification
of a marked and important kind;" and apparently he would extend the
same view to the wings of bats and pterodactyles. This conclusion,
which implies great breaks or discontinuity in the series, appears to me
improbable in the highest degree.

Everyone who believes in slow and gradual evolution, will of course
admit that specific changes may have been as abrupt and as great as
any single variation which we meet with under nature, or even under
domestication. But as species are more variable when domesticated or
cultivated than under their natural conditions, it is not probable that
such great and abrupt variations have often occurred under nature, as
are known occasionally to arise under domestication. Of these latter
variations several may be attributed to reversion; and the characters
which thus reappear were, it is probable, in many cases at first
gained in a gradual manner. A still greater number must be called
monstrosities, such as six-fingered men, porcupine men, Ancon sheep,
Niata cattle, etc.; and as they are widely different in character from
natural species, they throw very little light on our subject. Excluding
such cases of abrupt variations, the few which remain would at best
constitute, if found in a state of nature, doubtful species, closely
related to their parental types.

My reasons for doubting whet