BI-PARENTAL REPRODUCTION 7o 



130. It is unnecessary to multiply examples. " Reversion 

 is not a rare event, depending on some unusual or favourable 

 combination of circumstances, but occurs so regularly with 

 crossed animals and plants, and so frequently with uncrossed 

 breeds, that it is evidently an essential part of the principles 

 of inheritance." J The variability of crossed breeds, on which 

 so many writers have commented, will in almost all instances 

 be found to be of a regressive nature. 2 A little thought will 

 show that the " regression towards mediocrity " which occurs 

 when individuals of the same variety breed is a phenomenon 

 of the same kind that it is reversion towards the ancestry. 

 It matters not that in uncrossed species the reversion is 

 generally to the immediate ancestry. If unchecked by selec- 

 tion if will be towards a remoter ancestry in succeeding 

 generations. Nothing in nature is more certain than that 

 the function of the bi-parental reproduction is to produce 

 regressive variation ; and nothing in science is more remark- 



of bi-parental reproduction. But when individuals of different varieties 

 interbreed, the characters in which they differ are useful, since they have 

 been evolved by Natural Selection from the ancestral type. It is there- 

 fore an advantage that they should be both preserved by segregation in 

 the offspring. Varieties so seldom interbreed in nature that this 

 tendency to segregation as regards varietal characters can hardly have 

 arisen through the operation of Natural Selection. On the other hand, 

 if an individual varies greatly from the rest of his kind he constitutes a 

 variety by himself. If he survives his variation is probably useful. It 

 would be an advantage to preserve it. When one individual is a sport 

 (e. g. Ancon and Manchamp sheep, albinos) the offspring tend to inherit 

 from one or other parent exclusively. It is possible, then, that this 

 tendency to exclusive inheritance in the descendants of sports is a product 

 of Natural Selection, a real phase of evolution. Herein perhaps we have 

 the explanation of the fact that varietal characters tend to segregate 

 in the descendants of cross-breeds. In view, however, of the rarity of 

 great and useful variations even this explanation appears far-fetched. 

 That very dissimilar parents tend to have offspring in whom the inherit- 

 ance is exclusive is, therefore, probably no more than an accidental 

 result of conditions in the germ-plasm which are unknown to us. 



To sum up; the unions of very dissimilar individuals (e. g, those 

 belonging to separate genera) are usually sterile ; the unions of less 

 dissimilar individuals (e. g. those belonging to separate varieties) usually 

 result in exclusive inheritance or reversion to the ancestral type ; the 

 unions of still less dissimilar individuals (e. g. those within the same 

 variety) usually result in blended inheritance, when there is generally 

 regression towards the common varietal type that is regression to 

 mediocrity. (See Appendix B.) 



1 Animals and Plants, vol. ii., p. 368. 



2 There are some apparent exceptions. (See 162-3.) Moreover when 

 there is exclusive inheritance, or when this or that character is identical 

 in both crossed species, progressive variations are possible. 



