natural selection。 But I am bound to confess； that； with all my faith in this principle； I should never have anticipated that natural selection could have been efficient in so high a degree； had not the case of these neuter insects convinced me of the fact。 I have； therefore； discussed this case； at some little but wholly insufficient length； in order to show the power of natural selection； and likewise because this is by far the most serious special difficulty； which my theory has encountered。 The case； also； is very interesting； as it proves that with animals； as with plants； any amount of modification in structure can be effected by the accumulation of numerous； slight； and as we must call them accidental； variations； which are in any manner profitable； without exercise or habit having come into play。 For no amount of exercise； or habit； or volition； in the utterly sterile members of a community could possibly have affected the structure or instincts of the fertile members； which alone leave descendants。 I am surprised that no one has advanced this demonstrative case of neuter insects； against the well…known doctrine of Lamarck。
Summary。 I have endeavoured briefly in this chapter to show that the mental qualities of our domestic animals vary； and that the variations are inherited。 Still more briefly I have attempted to show that instincts vary slightly in a state of nature。 No one will dispute that instincts are of the highest importance to each animal。 Therefore I can see no difficulty； under changing conditions of life； in natural selection accumulating slight modifications of instinct to any extent； in any useful direction。 In some cases habit or use and disuse have probably come into play。 I do not pretend that the facts given in this chapter strengthen in any great degree my theory； but none of the cases of difficulty； to the best of my judgment； annihilate it。 On the other hand； the fact that instincts are not always absolutely perfect and are liable to mistakes； that no instinct has been produced for the exclusive good of other animals； but that each animal takes advantage of the instincts of others； that the canon in natural history； of 'natura non facit saltum' is applicable to instincts as well as to corporeal structure； and is plainly explicable on the foregoing views； but is otherwise inexplicable； all tend to corroborate the theory of natural selection。
This theory is； also； strengthened by some few other facts in regard to instincts； as by that common case of closely allied； but certainly distinct； species； when inhabiting distant parts of the world and living under considerably different conditions of life； yet often retaining nearly the same instincts。 For instance； we can understand on the principle of inheritance； how it is that the thrush of South America lines its nest with mud； in the same peculiar manner as does our British thrush： how it is that the male wrens （Troglodytes） of North America； build 'cock…nests；' to roost in； like the males of our distinct Kitty…wrens； a habit wholly unlike that of any other known bird。 Finally； it may not be a logical deduction； but to my imagination it is far more satisfactory to look at such instincts as the young cuckoo ejecting its foster…brothers； ants making slaves； the larvae of ichneumonidae feeding within the live bodies of caterpillars； not as specially endowed or created instincts； but as small consequences of one general law； leading to the advancement of all organic beings； namely； multiply； vary； let the strongest live and the weakest die。
Chapter 8 … Hybridism
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。 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。 The importance of the fact that hybrids are very generally sterile； has； I think； been much underrated by some late writers。 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。 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。
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。
Pure species have of course their organs of reproduction in a perfect condition； yet when intercrossed they produce either few or no offspring。 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。 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。 This distinction is important； when the cause of the sterility； which is common to the two cases； has to be considered。 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。
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。
First； for the sterility of species when crossed and of their hybrid offspring。 It is impossible to study the several memoirs and works of those two conscientious and admirable observers； K？lreuter and G？rtner； who almost devoted their lives to this subject； without being deeply impressed with the high generality of some degree of sterility。 K？lreuter makes the rule universal； but then he cuts the knot； for in ten cases in which he found two forms； considered by most authors as distinct species； quite fertile together； he unhesitatingly ranks them as varieties。 G？rtner； also； makes the rule equally universal； and he disputes the entire fertility of K？lreuter's ten cases。 But in these and in many other cases； G？rtner is obliged carefully to count the seeds； in order to show that there is any degree of sterility。 He always compares the maximum number of seeds produced by two species when crossed and by their hybrid offspring； with the average number produced by both pure parent…species in a state of nature。 But a serious cause of error seems to me to be here introduced： a plant to be hybridised must be castrated； and； what is often more important； must be secluded in order to prevent pollen being brought to it by insects from other plants。 Nearly all the plants experimentised on by G？rtner were potted； and apparently were kept in a chamber in his house。 That these processes are often injurious to the fertility of a plant cannot be doubted； for G？rtner gives in his table about a score of cases of plants which he castrated； and artificially fertilised with their own pollen； and （excluding all cases such as the Leguminosae； in which there is an acknowledged difficulty in the manipulation） half of these twenty plants had their fertility in some degree impaired。 Moreover； as G？rtner during several years repeatedly crossed the primrose and cowslip； which we have such good reason to believe to be varieties； and only once or twice succeeded in getting fertile seed； as he found the common red and blue pimpernels （Anagallis arvensis and coerulea）； which the best botanists rank as varieties； absolutely sterile together； and as he came to the same conclusion in several other analogous cases； it seems to me that we may well be permitted to doubt whether many other species are really so sterile； when intercrossed； as G？rtner believes。
It is certain； on the one hand； that the sterility of various species when crossed is so different in degree and graduates away so insensibly； and； on the other hand； that the fertility of pure species is so easily affected by various circumstances； that for all practical purposes it is most difficult to say where perfect fertility ends and sterility begins。 I think no better evidence of this can be required than that the two most experienced observers who have ever lived； namely； K？lreuter and G？rtner； should have arrived at diametrically opposite conclusions in regard to the very same species。 It is also most instructive to compare but I have not space here to enter on details the evidence advanced by our best botanists on the question whether certain doubtful forms should be ranked as species or varieties； with the evidence from fertility adduced by different hybridisers； or by the same author； from experiments made during different years。 It can thus be shown that neither sterility nor fertility affords any clear distinction between species and varieties； but that the evidence from this source graduates away； and is doubtful in the same degree as is the evidence derived from other constitutional and structural differences。
In regard to the sterility of hybrids in successive generations； though G？rtner was enabled to rear some hybrids； carefully guarding them from a cross with either pure parent； for six or seven； and in one case for ten generations； yet he asserts positively that their fertility never increased； but generally greatly decreased。 I do not doubt that this is usually the case； and that the fertility often suddenly decreases in the first few generations。 Nevertheless I believe that in all these experiments the fertility has been diminished by an independent cause； namely； from close interbreeding。 I have collected so large a body of facts； showing that close interbreeding lessens fertility； and； on the other hand； that an occasional cross with a distinct individual or varie