It is a fact, which cannot be denied, that in numberless instances the young of a creature differ more or less from the parents and likewise among themselves. Why it should be so has been in part explained. This is the _Variability_, without which Natural Selection could never have been thought of, because without differences there would have been nothing to select. But this Variability being granted, the Darwinian theory becomes possible--becomes quite capable of referring back the elephant and the pig, for instance, to the same ancestry. The difference between progenitors and their immediate offspring are, it is true, comparatively slight. It would, indeed, be a prodigious birth if one family contained at once a young monkey, a little pig, a big donkey, and a great goose; but it is obviously possible that any amount of unlikeness may be found between the descendants of common ancestors, if we are not confined to the differences of a single generation, but are allowed to multiply them through as many thousands as we require. Say that two race-courses differ in length by one yard; multiply that difference 1760 times and they will then differ by a whole mile. If, on leaving this Lecture-room, you found the trees--which half-an-hour ago were bare and leafless--clothed with summer verdure, your gardens blooming with a wealth of roses, your orchards laden with autumnal fruits, you would scarcely credit your senses; and yet, when the requisite number of half-hours, reckoned by days and months, shall have elapsed, you will greet these wonderful changes as perfectly natural and nothing to be wondered at. In a dissolving view that is well managed, Alpine peak and glacier-pass melt imperceptibly into some tall cathedral and sunshiny market-place. The two scenes are wholly unlike, and yet it is contrived that at no moment should the passage from one to the other be discernibly abrupt. Is it not possible then to conceive that through an immense multitude of generations the form of an ape might be derived from the form of a fish? We do not mean to say that this has actually happened, but supposing the descendant of the fish to vary continuously in the direction of the ape-like form, the result would be intelligible enough. What, then, is there to determine variation in any particular direction, and what limits are there, if any, to the system of interminable change which the principle of variation seems to involve?