Origin of Species Variorum

organisation, and hence we need not be surprised that their sterility, though in some degree variable, does not diminish; it is even apt to increase, this being generally the result, as before explained, of too close interbreeding. The above view of the sterility of hybrids being caused by two different constitutions being confounded into one has lately been strongly maintained by Max Wichura; but it must be owned that the sterility (as will be immediately explained) which affects the offspring of dimorphic and trimorphic plants, when individuals belonging to the same form are united, makes this view rather doubtful. It should, however, be borne in mind that the sterility of these plants has been acquired for a special purpose, and may differ in origin from that of hybrids.

It must .. be owned that we cannot understand, .. on the above or any other view, several facts with respect to the sterility of hybrids; for instance, the unequal fertility of hybrids produced from reciprocal crosses; or the increased sterility in those hybrids which occasionally and exceptionally resemble closely either pure parent. Nor do I pretend that the foregoing remarks go to the root of the matter; no explanation is offered why an organism, when placed under unnatural conditions, is rendered sterile. All that I have attempted to show, is that in two cases, in some respects allied, sterility is the common result,—in .. the one case from the conditions of life having been disturbed, in the other case from the organisation or constitution having been disturbed by two organisations being compounded into one.

A similar parallelism apparently extends to an allied yet very different class of facts. It is an old and almost universal belief, founded .. on a considerable body of evidence, that slight changes in the conditions of