Many years before Darwin's celebrated theory came to light, the question whether the repeated changes in animal and vegetable creation were the effects of changes in the external conditions of organic life, had been discussed among many palæontologists.

The solution of this question having to be sought for only within those deposits the Fauna of which is so nearly allied to that of present times that we can hope for a rather clearer idea of the condition in which these extinct forms were living, I have, a long time ago, been gathering a store of materials for the history of the Vienna Tertiaries, intending, in obedience to Bacon's precept—“Non disputando adversarium, sed opere naturam vincere.”

I have now to treat this matter,—first, in its stratigraphical aspect, describing the changes in external physical circumstances, then as a question of palæontology, inquiring into the action of those changes on the organic being coeval with them. I have previously had occasion to publish some result of my investigations in both these directions (see Acad. Proc. 1860, vol. xxxix. p. 158-166); and among the most important of them I may number the separation of the Vienna tertiaries into an Alpine and Extra-Alpine basin; the statement of repeated upheavings, of coevality of the apparently different deposits of Nussdorf, Grand, Baden, &c.; and lastly, the distinction of several successive Faunae of terrestrial mammalia.