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Show 258 HYBRIDISM. CHAP. VIII. By a reciprocal cross between ~wo species,. I mean the case, for instance, of a stalhon-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 b~en reciprocally crossed. There is of~en the _widest possible difference in the facility of making remprocal crosses. Such cases are highly important, for th~y prove that the capacity in any two spe?ies to cro~s Is o~ten completely independent of therr sys~ematiC affin1ty~ or. of any recognisable difference in therr 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 r~sult of reciprocal crosses between the same .two sp~mes was long ago observed by Kolreuter. To give an Instance: Mirabilis jalappa can easily be fertilised by the pollen of M. longi:flora, and the hybrids thus produced are sufficiently fertile ; but Kolreuter tried more than two hundred times, during eight following years, to fertilise reciprocally M. longi:flora 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. Gartner, 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, tho~gh of course compounded of the ~ery same two speCies, the one species having first been used as the f~~her_and then as the mother, generally differ in fertility 1n a small, and occasionally in a high degree. . Several other singular rules could be given from CHAP. VIII. LAWS OF STERILITY. 259 Gartner: 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 close! y 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 zen> 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 ~acility of making a :first cross between any two species Is not always governed by their systematic affinity or |
