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Show 228 HYBRIDISM, [CHAP. VIII. with a female-ass, and then a male-ass with a ma:e: th!fe twos ecies may then be said to have ?een ~emproca .Y crossep d . Th ere 1. s often the widest possibleS d ifhf erence ln the facility of making reciprocal crosses. uc . ca~es are highly important, for they prove that the ~apacity In any two species to cross is often completely ;ndepe~dent of their systmnatic affinity, or of any recognisable difference in their whole organisation. On the other ~and~ these cases clearly show that the capacity for crossn;g 1s connected with constitutional differences Imperce:J?tlb~e by us, and confined to the reproductive system. This difference in the result of reciprocal crosses between the sam~ two species was long ago observed by l~olreuter ... To give an instance: Mirabilis jalappa can eas1ly ?e fertihsed by the pollen of M. longifiora, and .. the hybr~ds thus produced are sufficiently fertile; but IColreute~ tried 1nore than ~:Vo hundred times, durb;g eigh~ followmg years, to _fertlhse reciprocally M. longiflora w1th tbe pollen of ¥·. Jalappa, and utterly failed. Several other equally striking ca~es could be given. Thure~ has ~bserved the same fact With certain sea-weeds or FuCl. Gartner, moreover, found th~t this difference of facility in making reciprocal crosses 1s extremely common in a lesser degree. l-Ie has obscryed it even between forms so closely related (as Matthiola annua and glabra) that many botanists rank them o~ly as varieties. It is also a remarkable fact that hybrids raised from reciprocal crosses, though of course ~ompou~ded of the very same two species, the one species hav1ng first been used as the father and then as the mother, generally differ in fertility in a small, and occasionally in a high degree. . f G'" t Several other singular rules could be given rom ar - ner: for instance, some species have a ren;arkable power of crossing with other species ; othe: spem~s of th~ s~me genus have a rernark.able power of Impressing thmr likeness on their hybrid offspring ; but these two po~ers do not at all necessarily go together. There ar~ certain .hybrids which instead of having, as is usual, an Intermediate character between their two parents, always closely resemble o:ne o.f them; and such hybrids, though externally CHAP. VIII.) COMPARED WITII GRAFTING. 229 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 s.ame 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 eminentl;y susceptible to favourable and unfavourable conditions, 1s 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 ext~r?al appea;ance either parent. And lastly, that the facihty 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 o: the mother, .there is generally some d1fference, and occasionally the w1dest 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 specie~ have b~en endowed wit~ sterility simply to prevent thmr becoming confounded 1n nature ? I think not. For why should the sterility be so extremely different. in degree, when va:ious species are crossed, all of whwh we must suppose 1t would be equally important to keep f~~m blending together ? Why should the deoTee of ster1hty be innately variable in the individuals o£5 the |
