own part I could no more admit the former proposition than I could admit that the planets move in their courses, and that a stone falls to the ground, not through the intervention of the secondary and appointed law of gravity, but from the direct volition of the Creator. {516} The discussion on the three species of _Rhinoceros_ which also occurs in the Essay of 1842, p. 48, was omitted in Ch. XIV of the _Origin_, Ed. i. Before concluding it will be well to show, although this has incidentally appeared, how far the theory of common descent can legitimately be extended{517}. If we once admit that two true species of the same genus can have descended from the same parent, it will not be possible to deny that two species of two genera may also have descended from a common stock. For in some families the genera approach almost as closely as species of the same genus; and in some orders, for instance in the monocotyledonous plants, the families run closely into each other. We do not hesitate to assign a common origin to dogs or cabbages, because they are divided into groups analogous to the groups in nature. Many naturalists indeed admit that all groups are artificial; and that they depend entirely on the extinction of intermediate species. Some naturalists, however, affirm that though driven from considering sterility as the characteristic of species, that an entire incapacity to propagate together is the best evidence of the existence of natural genera. Even if we put on one side the undoubted fact that some species of the same genus will not breed together, we cannot possibly admit the above rule, seeing that the grouse and pheasant (considered by some good ornithologists as forming two families), the bull-finch and canary-bird have bred together. {517} This corresponds to a paragraph in the _Origin_, Ed. i. p. 483, vi. p. 662, where it is assumed that animals have descended "from at most only four or five progenitors, and plants from an equal or lesser number." In the _Origin_, however, the author goes on, Ed. i. p. 484, vi. p. 663: "Analogy would lead me one step further, namely, to the belief that all animals and plants have descended from some one prototype."