The Origin of Species: A Variorum Text

CHAPTER VIII.

CHAPTER VIII. I:I IX. 2 HYBRIDISM. 3 Distinction between the sterility of first crosses and of hybridsSterility various in degree, not universal, affected by close interbreeding , removed by domestication-Laws governing the sterility of hybrids-Sterility not a special endowment, but incidental on other differences-Causes of the sterility of first crosses and of hybrids-Parallelism between the effects of changed conditions of life and crossing-Fertility of varieties when crossed and of their mongrel offspring not universal-Hybrids and mongrels compared independently of their fertility-Summary. ]:d differences, not accumulated by natural selection-Causes/life and of crossing-Dimorphism and trimorphism-Fertility of varieties 4 THE view generally entertained by naturalists is that species, when intercrossed, have been specially endowed with the quality of sterility, in order to prevent the confusion of all organic forms. 4:e with sterility, in order to prevent their confusion. 4:/ view commonly entertained 5 This view certainly seems at first probable, for species within the same country could hardly have kept distinct had they been capable of crossing freely. 5:e first highly probable/have been kept/of freely crossing. 5:/ species living together could 6 The importance of the fact that hybrids are very generally sterile, has, I think, been much underrated by some late writers. 6:d that first crosses between distinct species and hybrids 6:[e] 7 On the theory of natural selection the case is especially important, inasmuch as the sterility of hybrids could not possibly be of any advantage to them, and therefore could not have been acquired by the continued preservation of successive profitable degrees of sterility. 7:d important, inasmuch as this sterility can hardly have been increased by the continued preservation of successive, profitable degrees of sterility. 7:e The subject is in many ways important for us, more especially 424 as the sterility of species when first crossed, and that of. their hybrid offspring, cann?t have been acquired by the. ~ontinued preservation of successive, profitable degrees of sterility, 7:1 acquired, as I shall show, by the preservation of successive profitable 8 I hope, however, to be able to show that sterility is not a specially acquired or endowed quality, but is incidental on other acquired differences. 8:c other acquired and little known differences. 8:e But to this subject I shall have to return, and I hope ultimately to show that this sterility is neither a specially acquired nor endowed quality, but is incidental on other acquired and littleknown differences in the reproductive systems of the parentspecies . 8:1 It is an incidental result of differences in the reproductive systems of the parent-species. 9 In treating this subject, two classes of facts, to a large extent fundamentally different, have generally been confounded together ; namely, the sterility of two species when first crossed, and the sterility of the hybrids produced from them. 9:/ confounded; namely, the sterility of species 10 Pure species have of course their organs of reproduction in a perfect condition, yet when intercrossed they produce either few or no offspring. II Hybrids, on the other hand, have their reproductive organs functionally impotent, as may be clearly seen in the state of the male element in both plants and animals; though the organs themselves are perfect in structure, as far as the microscope reveals. I i td though the formative organs 12 In the first case the two sexual elements which go to form the embryo are perfect; in the second case they are either not at all developed, or are imperfectly developed. 13 This distinction is important, when the cause of the sterility, which is common to the two cases, has to be considered. 14 The distinction has probably been slurred over, owing to the sterility in both cases being looked on as a special endowment, beyond the province of our reasoning powers. I4:/ distinction probably has been 15 The fertility of varieties, that is of the forms known or believed to have descended from common parents, when intercrossed , and likewise the fertility of their mongrel offspring, is, on my theory, of equal importance with the sterility of species; for it seems to make a broad and clear distinction between varieties and species. I5:C offspring, is, with reference to my 425 I5:/ to be descended from common parents, when crossed, and likewise I5. I :d[~] Degrees of Sterility.16 First, for the sterility of species when crossed and of their hybrid offspring. I6:d[No ~] 17...