Finally, then, although we are as ignorant of the precise cause of the sterility of first crosses and of hybrids as we are why animals and plants removed from their natural conditions become sterile, yet the facts given in this chapter do not seem to me opposed to the belief that species aboriginally existed as varieties.

On the absence of intermediate varieties at the present day -- On the nature of extinct intermediate varieties; on their number -- On the lapse of time, as inferred from the rate of denudation and of deposition number -- On the lapse of time as estimated by years -- On the poorness of our palaeontological collections -- On the intermittence of geological formations -- On the denudation of granitic areas -- On the absence of intermediate varieties in any one formation -- On the sudden appearance of groups of species -- On their sudden appearance in the lowest known fossiliferous strata -- Antiquity of the habitable earth.

In the sixth chapter I enumerated the chief objections which might be justly urged against the views maintained in this volume.