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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