CHAPTER XXVIII 

 VARIATION IN DOMESTIC ANIMALS 



Half a century ago when Darwin found it necessary to demonstrate 

 the widespread existence of variation, he selected as his most convincing 

 evidence the variability which occurs among domesticated animals and 

 plants. In "The Variation of Animals and Plants under Domestication" 

 he has given us a masterly, and at the same time delightful, account of the 

 extreme variation which is exhibited by domesticated breeds of livestock. 

 Even today although we cannot accept the explanations which Darwin 

 offered to account for these variations, this treatise remains the best gen- 

 eral account of variation among farm animals and household pets. But 

 since Darwin's time the point of view has shifted from the question of the 

 occurrence of variation, now universally accepted as an established fact, 

 to the problem of the sources and causes of variation, a problem about 

 which we still have much to learn. 



The Sources of Variation. With regard to their relations to each 

 other and their specific causes, our knowledge of variation in domestic 

 varieties of animals is unfortunately considerably circumscribed. Since, 

 however, it has been demonstrated that variability among all living 

 beings arises from the same general sources, we may with confidence 

 state that among domestic animals, as among other living forms which 

 have been studied in greater detail, variations may be classified with 

 respect to source under three primary heads: somatic modifications, 

 germinal recombinations, and germinal alterations or mutations. More- 

 over, the behavior within these groups among farm animals is strictly 

 typical for the class in question. Somatic modifications arise from en- 

 vironmental causes, and they are merely transient; they leave no impres- 

 sion, whatever, on the germ-plasm. Variation by germinal recombina- 

 tions arises from amphimixis, and in domestic animals, we have a growing 

 body of evidence in support of the belief that such recombinations uni- 

 versally follow strictly the Mendelian law of segregation. Definite, 

 authentic cases of mutational changes in higher animals are exceedingly 

 rare, but those which we have leave no doubt that they involve single 

 locus alterations in the germinal material in a manner strictly analogous 

 to that of mutation in the fruit-fly. Of all these kinds of variation, there 

 are good isolated examples among domestic animals, but very often 

 there is a deplorable lack of detail about problems which offhand appear 



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