Not only are there good reasons against the acceptance of the exclusive operation of “Natural Selection” as the one means of specific origination, but there are difficulties in the way of accounting for such origination by the sole action of modifications which are infinitesimal and minute, whether fortuitous or not.

Arguments may yet be advanced in favour of the view that new species have from time to time manifested themselves with suddenness, and by modifications appearing at once (as great in degree as are those which separate Hipparion from Equus), the species remaining stable in the intervals of such modifications: by stable being meant that their variations only extend for a certain degree in various directions, like oscillations in a stable equilibrium. This is the conception of Mr. Galton, who compares the development of species with a many facetted spheroid tumbling over from one facet, or stable equilibrium, to another. The existence of internal conditions in animals corresponding with such facets is denied by pure Darwinians, but it is contended in this work, though not in this chapter, that something may also be said for their existence.

The considerations brought forward in the last two chapters, namely, the difficulties with regard to incipient and closely similar structures respectively, together with palæontological considerations to be noticed later, appear to point strongly in the direction of sudden and considerable changes.