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； in the one case from the conditions of life having been disturbed； in the other case from the organisation having been disturbed by two organisations having been compounded into one。
It may seem fanciful； but I suspect that a similar parallelism extends to an allied yet very different class of facts。 It is an old and almost universal belief； founded； I think； on a considerable body of evidence； that slight changes in the conditions of life are beneficial to all living things。 We see this acted on by farmers and gardeners in their frequent exchanges of seed； tubers； &c。； from one soil or climate to another； and back again。 During the convalescence of animals； we plainly see that great benefit is derived from almost any change in the habits of life。 Again； both with plants and animals； there is abundant evidence； that a cross between very distinct individuals of the same species； that is between members of different strains or sub…breeds； gives vigour and fertility to the offspring。 I believe； indeed； from the facts alluded to in our fourth chapter； that a certain amount of crossing is indispensable even with hermaphrodites； and that close interbreeding continued during several generations between the nearest relations； especially if these be kept under the same conditions of life； always induces weakness and sterility in the progeny。
Hence it seems that； on the one hand； slight changes in the conditions of life benefit all organic beings； and on the other hand； that slight crosses； that is crosses between the males and females of the same species which have varied and become slightly different； give vigour and fertility to the offspring。 But we have seen that greater changes； or changes of a particular nature； often render organic beings in some degree sterile； and that greater crosses； that is crosses between males and females which have become widely or specifically different； produce hybrids which are generally sterile in some degree。 I cannot persuade myself that this parallelism is an accident or an illusion。 Both series of facts seem to be connected together by some common but unknown bond； which is essentially related to the principle of life。
Fertility of Varieties when crossed； and of their Mongrel off…spring。 It may be urged； as a most forcible argument； that there must be some essential distinction between species and varieties； and that there must be some error in all the foregoing remarks； inasmuch as varieties； however much they may differ from each other in external appearance； cross with perfect facility； and yield perfectly fertile offspring。 I fully admit that this is almost invariably the case。 But if we look to varieties produced under nature； we are immediately involved in hopeless difficulties； for if two hitherto reputed varieties be found in any degree sterile together； they are at once ranked by most naturalists as species。 For instance； the blue and red pimpernel； the primrose and cowslip； which are considered by many of our best botanists as varieties； are said by G？rtner not to be quite fertile when crossed； and he consequently ranks them as undoubted species。 If we thus argue in a circle； the fertility of all varieties produced under nature will assuredly have to be granted。
If we turn to varieties； produced； or supposed to have been produced； under domestication； we are still involved in doubt。 For when it is stated； for instance； that the German Spitz dog unites more easily than other dogs with foxes； or that certain South American indigenous domestic dogs do not readily cross with European dogs； the explanation which will occur to everyone； and probably the true one； is that these dogs have descended from several aboriginally distinct species。 Nevertheless the perfect fertility of so many domestic varieties； differing widely from each other in appearance； for instance of the pigeon or of the cabbage； is a remarkable fact； more especially when we reflect how many species there are； which； though resembling each other most closely； are utterly sterile when intercrossed。 Several considerations； however； render the fertility of domestic varieties less remarkable than at first appears。 It can； in the first place； be clearly shown that mere external dissimilarity between two species does not determine their greater or lesser degree of sterility when crossed； and we may apply the same rule to domestic varieties。 In the second place； some eminent naturalists believe that a long course of domestication tends to eliminate sterility in the successive generations of hybrids； which were at first only slightly sterile； and if this be so； we surely ought not to expect to find sterility both appearing and disappearing under nearly the same conditions of life。 Lastly； and this seems to me by far the most important consideration； new races of animals and plants are produced under domestication by man's methodical and unconscious power of selection； for his own use and pleasure： he neither wishes to select； nor could select； slight differences in the reproductive system； or other constitutional difference correlated with the reproductive system。 He supplies his several varieties with the same food； treats them in nearly the same manner； and does not wish to alter their general habits of life。 Nature acts uniformly and slowly during vast periods of time on the whole organization； in any way which may be for each creature's own good； and thus she may； either directly； or more probably indirectly； through correlation； modify the reproductive system in the several descendants from any one species。 Seeing this difference in the process of selection； as carried on by man and nature； we need not be surprised at some difference in the result。
I have as yet spoken as if the varieties of the same species were invariably fertile when intercrossed。 But it seems to me impossible to resist the evidence of the existence of a certain amount of sterility in the few following cases； which I will briefly abstract。 The evidence is at least as good as that from which we believe in the sterility of a multitude of species。 The evidence is； also； derived from hostile witnesses； who in all other cases consider fertility and sterility as safe criterions of specific distinction。 G？rtner kept during several years a dwarf kind of maize with yellow seeds； and a tall variety with red seeds； growing near each other in his garden； and although these plants have separated sexes； they never naturally crossed。 He then fertilized thirteen flowers of the one with the pollen of the other； but only a single head produced any seed； and this one head produced only five grains。 Manipulation in this case could not have been injurious； as the plants have separated sexes。 No one； I believe； has suspected that these varieties of maize are distinct species； and it is important to notice that the hybrid plants thus raised were themselves perfectly fertile； so that even G？rtner did not venture to consider the two varieties as specifically distinct。
Girou de Buzareingues crossed three varieties of gourd； which like the maize has separated sexes； and he asserts that their mutual fertilization is by so much the less easy as their differences are greater。 Ho