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THE AMERICAN NATURALIST [Vol. XLVI1 



the members of natural species is a result of alternative inherit- 

 ance of contrasted parental characters, no less than the typical 

 eases of Mendelism. If inheritance were not alternative, heter- 

 ism would not be maintained. Each species or separate group 

 of interbreeding individuals would gradually decline into a gen- 

 eral uniform average. Under conditions of normal interbreed- 

 ing among the representatives of different lines of descent there 

 is no such tendency to uniformity. The offspring of the same 

 parents differ normally among themselves in the same ways that 

 the parents and ancestors have differed. Though each individ- 

 ual can bring into expression only one set of differences, other 

 ancestral characters are likely to reappear in later generations. 

 It is only by special methods of breeding in single or narrow 

 lines of descent that conditions of uniform heredity can be es- 

 tablished. 



Alternative Expression Instead of Alternative Trans- 

 mission 



That characters are transmitted without being brought into 

 expression is one of the best known facts of heredity, for which 

 the theory of Mendelism makes no adequate provision. The idea 

 of alternative expression of characters accommodates the numer- 

 ical data of Mendelism as well as the idea of alternative trans- 

 mission, and is in far better accord with other facts of variation. 

 Diversity in natural species, and reversions that arise in select 

 varieties and in hybrid stocks, afford adequate evidence for hold- 

 ing that alternative inheritance is due, not to alternative trans- 

 mission of characters, but to alternative expression. When the 

 facts of alternative expression are taken into account the theory 

 of alternative transmission becomes unnecessary. 3 



Coherence of Characters in Interspecific Hybrids 

 The contrasted characters of interspecific hybrids do not be- 

 have as independent Mendelian units, hut tend to remain more 

 or less united with others derived from the same parental stock. 

 This coherence of expression often interferes with the formation 

 of the combinations of characters according to the Mendelian 

 theory of independent segregation of discrete units. 4 



3 Cook, O. P., "Transmission Inheritance Distinct from Expression In- 

 heritance," Science, N. S., 25: 911, 1907. ^ ^ 



