“Although we may not be able to mark the precise boundary beyond which organic beings do not appear, it is certain that in all geological epochs, subsequent, at least, to that of the primary rocks, animals and plants have existed in successive families; they have been created, have lived their day, and by the operation of physical causes, have perished; while new races have been called into being, and in their turn have ceased to be, in order to give room to other families, requiring, perhaps, a different climate, and a new order of things.”