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Show 114 GOOD FROM CROSSING. C IIAP. )\ \ ' ]!. CHAPTER XY1I. · ON THE GOOD EFFECTS OF CHOSSING, · AND ON THE EVlL EFFECTS OF CLOSE INTEBBREEDING. JJEFINITION OF CI,OSE JN'rERBUEEDING- Al:G~ffiN'l'A'l'IO:-< OF MORBID TENDENC II·:~GEl\ TERAL EVIDE"'CE ON THE GOOD EFFECTS DEIU\'l~D FJW1l CllOSt>ING, A.c'W ON 'I'll:" EVIL EFFEC'l'S :I!' HOM CLOSE INTEUBREEDING- CAT'fl ,E, CLOSELY INTEHBHED ; ll ALl' wn, D CA'l'TLE LONG KEP'l' IN 'l'l:IE SAME PARKS- ,'JlEEP- FALLOW-DEETI- lJOGSRABBITS _ PIGS _ JIIAN, ORIGIN OF IllS ABHORRENCB Ol!' JNCEST"COl'S lilA HllJA(:ER _FOWLS -PIGEONS_ J-ITVE-BEES- PLAN'l'S, fJENEI\AL CONSTDEILA'l'IOC\;; ON 1'1~ 1." BENEFI'rS DEIUVED FHOU CltOSSING - JlffiLONS, FHLl'l'-TREES, l'EAS, CAllHAGh~, WHEAT, AND FOREST-TREES- ON THE INCREASED SI~E OF JIYBlllD l'LAN'l'S, NO'I' EXCL{;Sf\'ELY DUE TO TllEIR STElULI'l'Y- ON CER'.rA IN PLAN'l'i:l WHICH El:'m m NOI>MALLY OH ABNORMALLY ARE SELF-IJIIPOTEN'l', lH: 'l' ARE FEH'l'ILE, BOTH ON Tlll': 1\IALE Al'\D FEJIIAI,E SIDE, WHEN CROSSED Wl'l'H DIS'I'I:NCT Th"'DIVID"C.ALS EI'l'lllil> Ole '!'liE SAME OR .ANOTIIER SPECIES- CONCL"CSION. THE gain in constitutional vigour, derive~ from an occa~ional cross between individuals of the same vanety, but belongmg to distinct families, or between distinct varieties, has not been so largely or so frequently discussed, as have tho evil effe.cts of too close interbreedino-. But the former point is the more Important of the two, inas~uch as the evidence is more decisive. The evil results from close interbreeding are difficult to detect, for they accumulate slowly, and d~ffer much in degree. with_ different species; whilst the good effects which almost mvanably follow a cross are from the first manifest. It should, however, be clearly understood that the advantage of close int~r~ree_ding, as far as the retention of character is concerned, IS mcbsputable and often outweighs the evil of a slight loss of constitution~! vigour. In relation t~ the subject of dome~tication, ~he whole question is of some importance, as too close mterbree~mg interferes with the improvement of old races, and especially with the formation of new ones. It is important as indirectly bearing on Hybridism ; and perhaps on the extinction of species, when any form has become so rare that only a few m· d'I VI' d ua ls· CnAP . XVIJ. EVIL FROM INTERBREEDING. 115 remain within a confined area. It bears in an important manner on the influence of free intercrossing, in obliterating individual differences, and thus giving uniformity of character to the individuals of the same mce or species; for if additional vigour and fertility be thus gained, the crossed offspring will multiply and prevail, and the ultimate result will be far greater than otherwise would ha,·e occurred. Lastly, the question is of high interest, as bearing on mankind. Hence I shall discuss this subject at full length. As the facts which prove the evil effects of close interbreeding are more copious, thouo·h less decisive, than those on the good effects of crossing, I shalt under each group of beings, begin with the former. There is no difficulty in defining what is meant by a cross; but this is by no means easy in regard to " breeding in and in " or " too close interbreeding," because, as we shall see, different species of animals are differently affected by the same degree of interbreeding. The pairing of a father and daughter, or mother and son, or brothers and sisters, if carried on during several generations, is the closest possible form of interbreeding. But some good judges, for instance Sir J. Sebrigbt, believe that the pairing of a brother and sister is closer than that of parents and children; for when the father is matched with his daughter he crosses, as is said, with only half his own blood. The consequences of close interbreeding carried on for too long a time, are, as is generally believed, loss of size, constitutional vio·our, and fertility, sometimes accompanied by a tendency to 0 malformation. Manifest evil does not usually follow from pairinothe nearest relations for two, three, or even four genera~ tions ; but several causes interfere with our detecting the evil -such as the deterioration being very gradual, and the difficulty of distinguishing between such direct evil and the inevitable augmentation of any morbid tendencies which may be latent or apparent in the related parents. On the other hand, the benefit from a cross, even when there has not been any very close interbreeding, is almost invariably at once conspicuous. There is reason to believe, and this was the opinion of that most experienced observer Sir J. Sebright.,1 that the evil effects of close interbreeding may be checked by the related individuals 1 ' The Art of Improving the Breed, &c.,' 1809, p. 16. I 2 |
