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Show 48 INHERITANCE. CllAP. XIII. proper character of the variety, or its reversion to a former state. vVe have seen in the last section that when two races or species are crossed there is the strongest t endency to the reappearance in the offspring of long-lost characters, possess.ed by neither parent nor immediate progenitor. \Vhen two '':lute, or red, or black pigeons, of well-established breeds, are umteJ, the offspring are almost sure to inherit the same colours ; ~ut when differently-coloured birds · are crossed, the opposed forces of inheritance apparently counteract each other, and the tende~cy which is inherent in both parents to proc.lucc slaty-blue offsprmg becomes predominant. So it is in several other cases. Bnt when, for instance, the ass is crossed with A. lndicus or with the horse -animals which have not striped lcgs,-ancl the hybrids have con~picuous stripes on their legs and even on their fa~es, all that can be said is, that an inherent tendency to reverswn is evolved through some disturbance in the organisation caused by the act of crossing. . . Another form of reversion is far commoner, mdeed 1s almost universal with the offspring from e:t cross, namely, to the characters proper to either pure parent-form. As a general rule, crossed offspring in the first generation are nearly intermediate between their parents, but the grandchildren and succeeding generations continually revert, in a greater or lesser degr~e, to one or both of their progenitors. Several authors have mamtaineu that hybrids and mongrels include all the characters of both parents, not fused together, but merely mingled in different proportions in different parts of the body ; or, as :N auclin 48 bas expressed it, a hybrid is a living mosaic-work, in which the eye cannot distinguish the discordant elements, so completely are they intermingled. We can hardly doubt that, in a certain sense, this is true, as when we behold in a hybrid the clements of both species segregating themselves into segments in the same Hower or fruit, by a process of self-attraction or self-affinity; this seareO'ation taking place either by seminal or by bud-propagation. b b "fi N audin further believes that the segregation of the two speci c elements or essences is eminently . liable to occur in the male and female reproductive matter ; and he thus explains the almost 48 'Nouvelles Archives du Museum,' tom. i. p. 151. c nAP. xnr. REVERSION. 49 universal tendency to reversion in successive hybrid generations. For thi~ would be th~ natural result of the union of pollen and ovules, m both of whiCh the elements of the same species had be~n s~gregated by self-affinity. If, on the other hand, pollen w~wh mcluded the elements of one species happened to unite :VIth ovu:es including the elements of the other species, the mtermed1ate or hybrid state would still be retained, and there would be no reversion. But it would, as I suspect, be more correct to s.ay .that the elements of both parent-species exist in every hybnd m a double state, namely, blended together and completely separate. How this is possible, and what the term specific essence or element may be supposed to express, I shall attempt to show jn the hypothetical chapter on pangenesis. But N auclin's view, as propounded by him, is not applicable to t~e . reappearance of characters lost long ago by variation ; and It IS hardly applicable to races or species which, after having been crossed at some former period with a distinct form and having since lost all traces of the cross, nevertheless occ~sionally yield an individual which reverts (as in the case of the great-great-grandchild of the pointer Sappho) to the crossing form. The m~st simple case o~ reversion, namely, of a hybrid or ~ong~el to Its grandparents, IS connected by an almost perfect senes with tl~e extreme case of a purely-bred race recovering characters w~ICh had been lost during many ages; and we are thus led to mfer that all the cases mm~t be related by some common bond. . Gartner believed that only those hybrid plants which are highly steri~e exhibit any tendency to reversion to their parentforms. It IS rash to doubt so good an observer, but this conclusion must I think be an error; and it may perhaps be accounted ~or by the nature of the genera observed by him, for he adm~ts that ~he tendency differs in different genera. The statement 1s also duectly contradicted by N audin's observations a~d by the notori?us fa~t that perfectly fertile mongrels exhi~ b1t th~ tendency m a high degree,--even in a higher degree accordmg to Gartner himself, than hybrids.49 ' Gartner further states that reversions rarely occur with • 9 'Bastarderzeugung,' s. 582, 438, &c. VOL. II. E |