Upon the ability of like to produce like rests the continuity of those plant groups, well exemplified by _Lycopodium Selago_ and the ginkgo, which, through all the changing panorama of the history of the plant world, have steadily produced individuals so close to the ancestral type as to be essentially indistinguishable from it. It is upon the possession of this ability that all the different races of plants depend for the unchanged perpetuation of their kind. And, as we shall presently see, it is also upon this very ability that the new forms that do arise, rely for holding fast to their differences. While it is true, then, that like tends to produce like, it is also and perhaps even more true that they do not precisely do so. In fact, they never do absolutely, and it is the degree of divergence from the type that different plants or animals exhibit, which is the measure of their ability to vary, or “produce something different.” Upon this capacity to vary, from whatever cause, rest all the changes which have occurred in the plant world, and, as we have seen in previous chapters, that has been by no means an insignificant affair. We know, in fact, that while one plant of _Lycopodium Selago_, than which scarcely any other now living has had greater opportunity to become fixed in its characters, is much like another, no two of them are actually identical. Nor are any two plants of the same species ever precisely alike, any more than two children, even of the same parents, are. The tendency for like to produce like is matched then, or sometimes exceeded, by an almost equally strong tendency to vary.