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. 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.