MEXDELISM AND OTHER METHODS OF DESCENT I9I 



three quarters of the individuals have this character of the dom- 

 inant parent, and one quarter the other character (recessive), 

 which did not appear at all in the first generation. It is this 

 definite difference of proportions in the two generations which 

 Professor Davenport has accepted as the basis of the Mendelian 

 principles of heredity, and as a demonstration of the theor}'- of 

 evolution by mutation. 



" We find that when two varieties that differ in some charac- 

 teristic are crossed it frequently happens that one only of the 

 two forms will reappear in the offspring (viz., the dominant 

 characteristic, of Mendel), and it will be little modified by the 

 presence, in that offspring, of the germ of the opposite charac- 

 teristic. This is in accordance with the theory that most char- 

 acteristics are, or may be resolved into, elementary units. . . . 

 Similarly, when a variety that has some new feature not pos- 

 sessed by the ancestor is crossed with that ancestor the off- 

 spring usuall}'- have the character fully developed (dominant). If 

 these offspring are crossed together the character is absent in a 

 small proportion only, on the average one-quarter, of their 

 offspring. . . ." 



" The fact that in crossing varieties their dissimilar characters 

 do not blend is important, since it supports the theory that such 

 characters first appear as they now are, fully formed. It indi- 

 cates that since evolution has advanced by the addition of new 

 characteristics it has advanced by steps or jumps. A new 

 species has not gradually arisen from an old one, but suddenly, 

 by mutation."^ 



THE NATURE OF EXPERIMENTS IN DESCENT. 



But if the facts of Mendelism are examined somewhat more 

 closely and in the light of modern knowledge of the peculiar 

 nature of the reproductive processes of the higher plants and 

 animals, it will be found that the definite mathematical relations 

 which appear in a Mendelian experiment arise from the methods 

 of reproduction rather than from the methods of inheritance. 

 Other interpretations are possible, which render entirely unneces- 



' Davenport, C. B., 1906. Report on Experimental Biology, Yearbook No. 

 5, Carnegie Institution of Washington, D. C, p. 94. 



