But, says the objector, there is no such world; there is no world that has thus been driven back, and depressed from one deep to a lower deep. Granted: but the same effect, an illustration of the same law, is produced equally, whether you take four worlds, all of the same magnitude, and plunge them _simultaneously_ into four different abysses, sinking by graduated distances one below another, or take one world and plunge it to the same distances _successively_. So in Geology, when men talk of substances in different stages, or of transitional states, they do not mean that they have watched the same individual _stratum_ or _phenomenon_, exhibiting states removed from each other by depths of many thousand years; how could they? but they have seen one stage in the case A, another stage in the case B. They take, for instance, three objects, the same (to use the technical language of logic) generically, though numerically different, under separate circumstances, or in different stages of advance. They are one object for logic, they are three for human convenience. So again it might seem impossible to give the history of a rose tree from infancy to age: how could the same rose tree, at the same time, be young and old? Yet by taking the different developments of its flowers, even as they hang on the same tree, from the earliest bud to the full-blown rose, you may in effect pursue this vegetable growth through all its stages: you have before you the bony blushing little rose-bud, and the respectable 'mediæval' full-blown rose.