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ally been endowed with this quality； in order to prevent their crossing and blending together in utter confusion。 The following rules and conclusions are chiefly drawn up from G？rtner's admirable work on the hybridisation of plants。 I have taken much pains to ascertain how far the rules apply to animals； and considering how scanty our knowledge is in regard to hybrid animals； I have been surprised to find how generally the same rules apply to both kingdoms。
It has been already remarked； that the degree of fertility； both of first crosses and of hybrids； graduates from zero to perfect fertility。 It is surprising in how many curious ways this gradation can be shown to exist； but only the barest outline of the facts can here be given。 When pollen from a plant of one family is placed on the stigma of a plant of a distinct family； it exerts no more influence than so much inorganic dust。 From this absolute zero of fertility； the pollen of different species of the same genus applied to the stigma of some one species； yields a perfect gradation in the number of seeds produced； up to nearly complete or even quite complete fertility； and； as we have seen； in certain abnormal cases； even to an excess of fertility； beyond that which the plant's own pollen will produce。 So in hybrids themselves； there are some which never have produced； and probably never would produce； even with the pollen of either pure parent； a single fertile seed： but in some of these cases a first trace of fertility may be detected； by the pollen of one of the pure parent…species causing the flower of the hybrid to wither earlier than it otherwise would have done； and the early withering of the flower is well known to be a sign of incipient fertilisation。 From this extreme degree of sterility we have self…fertilised hybrids producing a greater and greater number of seeds up to perfect fertility。
Hybrids from two species which are very difficult to cross； and which rarely produce any offspring； are generally very sterile； but the parallelism between the difficulty of making a first cross； and the sterility of the hybrids thus produced two classes of facts which are generally confounded together is by no means strict。 There are many cases； in which two pure species can be united with unusual facility； and produce numerous hybrid…offspring； yet these hybrids are remarkably sterile。 On the other hand； there are species which can be crossed very rarely； or with extreme difficulty； but the hybrids； when at last produced； are very fertile。 Even within the limits of the same genus； for instance in Dianthus； these two opposite cases occur。
The fertility； both of first crosses and of hybrids； is more easily affected by unfavourable conditions； than is the fertility of pure species。 But the degree of fertility is likewise innately variable； for it is not always the same when the same two species are crossed under the same circumstances； but depends in part upon the constitution of the individuals which happen to have been chosen for the experiment。 So it is with hybrids； for their degree of fertility is often found to differ greatly in the several individuals raised from seed out of the same capsule and exposed to exactly the same conditions。
By the term systematic affinity is meant； the resemblance between species in structure and in constitution； more especially in the structure of parts which are of high physiological importance and which differ little in the allied species。 Now the fertility of first crosses between species； and of the hybrids produced from them； is largely governed by their systematic affinity。 This is clearly shown by hybrids never having been raised between species ranked by systematists in distinct families； and on the other hand； by very closely allied species generally uniting with facility。 But the correspondence between systematic affinity and the facility of crossing is by no means strict。 A multitude of cases could be given of very closely allied species which will not unite； or only with extreme difficulty； and on the other hand of very distinct species which unite with the utmost facility。 In the same family there may be a genus； as Dianthus； in which very many species can most readily be crossed； and another genus； as Silene； in which the most persevering efforts have failed to produce between extremely close species a single hybrid。 Even within the limits of the same genus； we meet with this same difference； for instance； the many species of Nicotiana have been more largely crossed than the species of almost any other genus； but G？rtner found that N。 acuminata； which is not a particularly distinct species； obstinately failed to fertilise； or to be fertilised by； no less than eight other species of Nicotiana。 Very many analogous facts could be given。
No one has been able to point out what kind； or what amount； of difference in any recognisable character is sufficient to prevent two species crossing。 It can be shown that plants most widely different in habit and general appearance； and having strongly marked differences in every part of the flower； even in the pollen； in the fruit； and in the cotyledons； can be crossed。 Annual and perennial plants； deciduous and evergreen trees； plants inhabiting different stations and fitted for extremely different climates； can often be crossed with ease。
By a reciprocal cross between two species； I mean the case； for instance； of a stallion…horse being first crossed with a female…ass； and then a male…ass with a mare： these two species may then be said to have been reciprocally crossed。 There is often the widest possible difference in the facility of making reciprocal crosses。 Such cases are highly important； for they prove that the capacity in any two species to cross is often completely independent of their systematic affinity； or of any recognisable difference in their whole organisation。 On the other hand； these cases clearly show that the capacity for crossing is connected with constitutional differences imperceptible by us； and confined to the reproductive system。 This difference in the result of reciprocal crosses between the same two species was long ago observed by K？lreuter。 To give an instance： Mirabilis jalappa can easily be fertilised by the pollen of M。 longiflora； and the hybrids thus produced are sufficiently fertile； but K？lreuter tried more than two hundred times； during eight following years； to fertilise reciprocally M。 longiflora with the pollen of M。 jalappa； and utterly failed。 Several other equally striking cases could be given。 Thuret has observed the same fact with certain sea…weeds or Fuci。 G？rtner； moreover； found that this difference of facility in making reciprocal crosses is extremely common in a lesser degree。 He has observed it even between forms so closely related （as Matthiola annua and glabra） that many botanists rank them only as varieties。 It is also a remarkable fact； that hybrids raised from reciprocal crosses； though of course compounded of the very same two species； the one species having first been used as the father and then as the mother； generally differ in fertility in a small； and occasionally in a high degree。
Several other singular rules could be give