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THE AMERICAN NATURALIST [Vol. XLIII 



ism. But the principle will probably be found to apply 

 also where there is presence of an abnormal dominant 

 character. We do not know many such characters at 

 present, but susceptibility to rust in wheat and congenital 

 cataract in man may be cited as approaching our meaning. 



The instances where the presence or absence of evil 

 qualities is brought to notice through their isolation 

 by inbreeding are few in number, however, and can not 

 account for the large number of cases where there is a 

 loss of vigor by this means. 



Let us consider just what this deterioration, so-called, 

 is. Does it represent an actual degeneration in heredi- 

 tary characters? In general it does not. Darwin's ex- 

 periments consisted mainly in comparing heights of 

 plants. His measure of vigor, then, is a measure of 

 rapidity and amount of cell division. In no cases does 

 he speak of losses of characters, and seldom of disease. 

 Even where disease appeared, it usually appeared alike 

 in inbred and cross-bred plants. In my own experiments, 

 I have observed some twenty-five families of maize in- 

 bred for two generations, and a lesser number through 

 the fourth generation, and have not found a single sign 

 of degeneration. In all characters of stalk, leaves, roots, 

 male inflorescence, female inflorescence, and mature seed, 

 the plants were normal. It is merely in the matter of 

 size of plant and ear, and thereby yield, that the plant 

 compares unfavorably with cross-bred plants. 



Further, there is no continuous decline in yield as 

 should be expected in actual degeneration. There is 

 somewhat greater difference between fourth generation 

 inbred and cross-bred plants than there is between those 

 of the second generation. This last fact may be ex- 

 plained by considering the frequency with which heter- 

 ozygotes in certain characters are selected even in the 

 most careful work. Not all character pairs can be kept 

 under observation, and from the mere fact of its being 

 inbred we can not presume that the isolation of a homo- 

 zygous strain is complete. The past summer one of our 



