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THE AMERICAN NATURALIST [Vol. XLIII 



all pointed to the truth of the dictum "Nature abhors 

 perpetual self-fertilization," which, by the way, simply 

 corroborated the results that Knight had obtained a half 

 century before. That there are important exceptions 

 to this rule was recognized, however, and Hays, 1 arguing 

 from the case of wheat, suggested that it be changed to 

 read: " Nature abhors a radical change which would re- 

 quire species to cross in much closer or in much more 

 radical relationship than is their long established habit. ' ' 

 Each of these sayings is probably correct as far as it goes. 

 Nature does seem to have provided more for cross-fertil- 

 ization than self-fertilization. Yet the very fact that 

 all species do not cross-fertilize naturally, shows that 

 although cross-fertilization may be a desirable thing— 

 a thing to be provided for by nature— it does not follow 

 that inbreeding and decrease in vigor hold the relation 

 of cause and effect because they are often linked together. 



As a matter of fact, the data that we possess regarding 

 the supposed degeneration through inbreeding do admit 

 an entirely different explanation, an explanation more 

 compatible with contemporary knowledge. The hy- 

 pothesis, first suggested by Shull, 2 is that the danger 

 from self-fertilization in naturally cross-bred species may 

 be due simply to the isolation of biotypes. It is an 

 established fact, although the cause is unknown, that 

 crosses between nearly related types (in both animals and 

 plants) are usually more vigorous than either of the par- 

 ent ty*pes alone. Since inbreeding tends to isolate types 

 homozygous in their characters, these homozygous types, 

 coming from species naturally crossbred, are thus de- 

 prived of the stimulus which came through free intercross- 

 ing and appear to deteriorate. 



A little later the present writer 3 pointed out that a 



1 Hays, W. M. "Plant Breeding." Bull. IT. S. D. A. Div. Veg. Phy. & 

 Path. 29: 1901. 



2 Shull, G. H. "The Composition of a Field of Maize." Ann. Bpt. 

 Amer. Breeders' Assn. 4: 1908. 



•East, E. M. "Inbreeding in Corn," Ann. Ept. Conn. Agr. Exp. Sta- 

 tion, 1907-8. 



