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THE AMERICAN NATURALIST [Vol. LIV 



The observations of the writer on the enormous variability of 

 the F 2 generations arising from partially sterile F t generations 

 produced by crossing species, led him to suspect that such com- 

 binations might be the basis of a great deal of variability respon- 

 sible for evolution under domestication. A careful survey of the 

 evidence relating to the origin of modern horses, cattle, sheep,, 

 swine, dogs, guinea pigs, fowls, ducks, and geese on the one hand, 

 and varieties of wheat, corn, barley, oats, rye, apples, grapes, 

 roses and begonias on the other hand, shows that in every case 

 several related wild or semi-wild species exist which will cross 

 together and yield partially fertile offspring. Doubtless many 

 other species which have shown great improvement under domes- 

 tication, would be found to have wild relatives which behave sim- 

 ilarly, should they be investigated. Both the historical and the 

 experimental evidence, therefore, point to hybridization, and 

 particularly to species of hybridization, as the great single cause 

 of evolution under domestication. 



At the same time, one must not confuse evolution under domes- 

 tication with natural evolution. The outstanding biological fea- 

 ture characteristic of the varied groups of domestic animals and 

 of cultivated plants, is the perfect fertility within each group. 

 A marked peculiarity of the great majority of natural species is 

 their sterility with one another, the origin of which has long 

 been a stumbling block to writers on evolutionary biology. Our 

 own experimental evidence, as far as it goes, and observations on 

 domestic forms which presumably have originated from com- 

 binations of two or more wild species, yield not the slightest in- 

 dication of a tendency toward the production of segregates that 

 exhibit either incompatibility in crosses or sterility of the indi- 

 viduals produced by hybridization. 



E. M. East 



THE MEASUREMENT OF LINKAGE 

 Linkage is a name for that tendency sometimes shown by 

 genes to maintain in hereditary transmission their previous rela- 

 tions to each other. Thus if two linked genes, A and B, enter a 

 cross together, in the same gamete, they will oftener than not be 

 found together in the gametes formed by the eross-bred indi- 

 vidual. And if the same two genes enter the cross separately, 



