Now, from the working of Nature under, as it were, man’s guidance, we pass to the working of Nature when left to her own discretion. The work of Natural Selection is a very slow and secret work: the slowness of it veils the movement. As with the hour-hand of a tiny watch travelling but an inch in a day, there is progress which you cannot discern, there is change that can be marked and registered at intervals, though each successive moment and each successive movement seem to leave things exactly as they were. You have heard of the Greek simpleton who had been told that a raven lived three hundred years, and so bought one to see. We might live three thousand years instead of three hundred without being able to prove the theory of Natural Selection by actual observing. But when a group of most important observed facts can be explained consistently by this theory, and by none other, while no fact has been brought forward to make it inadmissible, it ought to be accepted till some theory can be produced equally unimpeachable and explanatory of a larger group of facts. Qualities are inherited; but with this peculiarity, that very generally, and sometimes of necessity, the inheritor comes into possession of the inherited quality at the same period of life at which it was acquired by the parent. As, for instance, the child of a gouty father, though it may be destined in old age to inherit the disease, is not born with the gout, any more than a calf is born with horns, or a cherry-tree produced covered with cherries. In the life of every creature there is not merely growth, but development. At every stage of life it is possible for some quality acquired by variation to be fixed by Natural Selection. But in the embryonic and earliest stages of development, variation is least likely to be of service to any creature. Such variations, therefore, will less often be selected than others, and if it be true that many species have a common ancestry, then it ought to be found that in their embryonic and earliest stages they resemble one another. This is precisely what we do find. Plants, the most remote in appearance and properties when full grown, differ but slightly in their cotyledons: the difference between the egg of a nightingale and the egg of an ostrich bears no proportion to the dissimilarity between the two birds when fully developed; nor by comparing the roe of a herring with the roe of a salmon could you possibly guess, before experience, how the full-grown fish would differ. But in the life of every human being there is a stage of development, at which the most sagacious physician could not distinguish him from the embryo of a snake, a lizard, a bird, or an ape[12]. Now, if the simplest embryonic forms of life were the progenitors of all existing forms, this is intelligible; but how else can it be explained?