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ertility in a small； and occasionally in a high degree。
Several other singular rules could be given from G？rtner： for instance； some species have a remarkable power of crossing with other species； other species of the same genus have a remarkable power of impressing their likeness on their hybrid offspring； but these two powers do not at all necessarily go together。 There are certain hybrids which instead of having； as is usual； an intermediate character between their two parents； always closely resemble one of them； and such hybrids； though externally so like one of their pure parent…species； are with rare exceptions extremely sterile。 So again amongst hybrids which are usually intermediate in structure between their parents； exceptional and abnormal individuals sometimes are born； which closely resemble one of their pure parents； and these hybrids are almost always utterly sterile； even when the other hybrids raised from seed from the same capsule have a considerable degree of fertility。 These facts show how completely fertility in the hybrid is independent of its external resemblance to either pure parent。
Considering the several rules now given； which govern the fertility of first crosses and of hybrids； we see that when forms； which must be considered as good and distinct species； are united； their fertility graduates from zero to perfect fertility； or even to fertility under certain conditions in excess。 That their fertility； besides being eminently susceptible to favourable and unfavourable conditions； is innately variable。 That it is by no means always the same in degree in the first cross and in the hybrids produced from this cross。 That the fertility of hybrids is not related to the degree in which they resemble in external appearance either parent。 And lastly； that the facility of making a first cross between any two species is not always governed by their systematic affinity or degree of resemblance to each other。 This latter statement is clearly proved by reciprocal crosses between the same two species； for according as the one species or the other is used as the father or the mother； there is generally some difference； and occasionally the widest possible difference； in the facility of effecting an union。 The hybrids； moreover； produced from reciprocal crosses often differ in fertility。
Now do these complex and singular rules indicate that species have been endowed with sterility simply to prevent their becoming confounded in nature？ I think not。 For why should the sterility be so extremely different in degree； when various species are crossed； all of which we must suppose it would be equally important to keep from blending together？ Why should the degree of sterility be innately variable in the individuals of the same species？ Why should some species cross with facility； and yet produce very sterile hybrids； and other species cross with extreme difficulty； and yet produce fairly fertile hybrids？ Why should there often be so great a difference in the result of a reciprocal cross between the same two species？ Why； it may even be asked； has the production of hybrids been permitted？ To grant to species the special power of producing hybrids； and then to stop their further propagation by different degrees of sterility； not strictly related to the facility of the first union between their parents； seems to be a strange arrangement。
The foregoing rules and facts； on the other hand； appear to me clearly to indicate that the sterility both of first crosses and of hybrids is simply incidental or dependent on unknown differences； chiefly in the reproductive systems； of the species which are crossed。 The differences being of so peculiar and limited a nature； that； in reciprocal crosses between two species the male sexual element of the one will often freely act on the female sexual element of the other； but not in a reversed direction。 It will be advisable to explain a little more fully by an example what I mean by sterility being incidental on other differences； and not a specially endowed quality。 As the capacity of one plant to be grafted or budded on another is so entirely unimportant for its welfare in a state of nature； I presume that no one will suppose that this capacity is a specially endowed quality； but will admit that it is incidental on differences in the laws of growth of the two plants。 We can sometimes see the reason why one tree will not take on another； from differences in their rate of growth； in the hardness of their wood； in the period of the flow or nature of their sap； &c。； but in a multitude of cases we can assign no reason whatever。 Great diversity in the size of two plants； one being woody and the other herbaceous； one being evergreen and the other deciduous； and adaptation to widely different climates； does not always prevent the two grafting together。 As in hybridisation； so with grafting； the capacity is limited by systematic affinity； for no one has been able to graft trees together belonging to quite distinct families； and； on the other hand； closely allied species； and varieties of the same species； can usually； but not invariably； be grafted with ease。 But this capacity； as in hybridisation； is by no means absolutely governed by systematic affinity。 Although many distinct genera within the same family have been grafted together； in other cases species of the same genus will not take on each other。 The pear can be grafted far more readily on the quince； which is ranked as a distinct genus； than on the apple； which is a member of the same genus。 Even different varieties of the pear take with different degrees of facility on the quince； so do different varieties of the apricot and peach on certain varieties of the plum。
As G？rtner found that there was sometimes an innate difference in different individuals of the same two species in crossing； so Sagaret believes this to be the case with different individuals of the same two species in being grafted together。 As in reciprocal crosses； the facility of effecting an union is often very far from equal； so it sometimes is in grafting； the common gooseberry； for instance； cannot be grafted on the currant； whereas the currant will take； though with difficulty； on the gooseberry。
We have seen that the sterility of hybrids； which have their reproductive organs in an imperfect condition； is a very different case from the difficulty of uniting two pure species； which have their reproductive organs perfect； yet these two distinct cases run to a certain extent parallel。 Something analogous occurs in grafting； for Thouin found that three species of Robinia； which seeded freely on their own roots； and which could be grafted with no great difficulty on another species； when thus grafted were rendered barren。 On the other hand； certain species of Sorbus； when grafted on other species； yielded twice as much fruit as when on their own roots。 We are reminded by this latter fact of the extraordinary case of Hippeastrum； Lobelia； &c。； which seeded much more freely when fertilised with the pollen of distinct species； than when self…fertilised with their own pollen。
We thus see； that although there is a clear and fundamental difference between the mere adhesi