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ught 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 variety increases fertility； that I cannot doubt the correctness of this almost universal belief amongst breeders。 Hybrids are seldom raised by experimentalists in great numbers； and as the parent…species； or other allied hybrids； generally grow in the same garden； the visits of insects must be carefully prevented during the flowering season： hence hybrids will generally be fertilised during each generation by their own individual pollen； and I am convinced that this would be injurious to their fertility； already lessened by their hybrid origin。 I am strengthened in this conviction by a remarkable statement repeatedly made by G？rtner； namely； that if even the less fertile hybrids be artificially fertilised with hybrid pollen of the same kind； their fertility； notwithstanding the frequent ill effects of manipulation； sometimes decidedly increases； and goes on increasing。 Now； in artificial fertilisation pollen is as often taken by chance （as I know from my own experience） from the anthers of another flower； as from the anthers of the flower itself which is to be fertilised； so that a cross between two flowers； though probably on the same plant； would be thus effected。 Moreover； whenever complicated experiments are in progress； so careful an observer as G？rtner would have castrated his hybrids； and this would have insured in each generation a cross with the pollen from a distinct flower； either from the same plant or from another plant of the same hybrid nature。 And thus； the strange fact of the increase of fertility in the successive generations of artificially fertilised hybrids may； I believe； be accounted for by close interbreeding having been avoided。
Now let us turn to the results arrived at by the third most experienced hybridiser； namely； the Hon。 and Rev。 W。 Herbert。 He is as emphatic in his conclusion that some hybrids are perfectly fertile as fertile as the pure parent…species as are K？lreuter and G？rtner that some degree of sterility between distinct species is a universal law of nature。 He experimentised on some of the very same species as did G？rtner。 The difference in their results may； I think； be in part accounted for by Herbert's great horticultural skill； and by his having hothouses at his command。 Of his many important statements I will here give only a single one as an example； namely； that 'every ovule in a pod of Crinum capense fertilised by C。 revolutum produced a plant； which （he says） I never saw to occur in a case of its natural fecundation。' So that we here have perfect； or even more than commonly perfect； fertility in a first cross between two distinct species。
This case of the Crinum leads me to refer to a most singular fact； namely； that there are individual plants； as with certain species of Lobelia； and with all the species of the genus Hippeastrum； which can be far more easily fertilised by the pollen of another and distinct species； than by their own pollen。 For these plants have been found to yield seed to the pollen of a distinct species； though quite sterile with their own pollen； notwithstanding that their own pollen was found to be perfectly good； for it fertilised distinct species。 So that certain individual plants and all the individuals of certain species can actually be hybridised much more readily than they can be self…fertilised！ For instance； a bulb of Hippeastrum aulicum produced four flowers； three were fertilised by Herbert with their own pollen； and the fourth was subsequently fertilised by the pollen of a compound hybrid descended from three other and distinct species： the result was that 'the ovaries of the three first flowers soon ceased to grow； and after a few days perished entirely； whereas the pod impregnated by the pollen of the hybrid made vigorous growth and rapid progress to maturity； and bore good seed； which vegetated freely。' In a letter to me； in 1839； Mr Herbert told me that he had then tried the experiment during five years； and he continued to try it during several subsequent years； and always with the same result。 This result has； also； been confirmed by