They seem to me to partake little of the character of
demonstration, and to have little weight in comparison with those in favour
of the power of natural selection, aided by the other agencies often
specified. I am bound to add, that some of the facts and arguments here
used by me, have been advanced for the same purpose in an able article
lately published in the "Medico-Chirurgical Review."
At the present day almost all naturalists admit evolution under some 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.
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