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.