214 COOK 



to have been prepotent in all the gametes of the next generation.^ 

 Descent with Combination o?' Averaging oj" Differences [Inter- 

 mediate). — Intermediate discriminate inheritance is a form of 

 descent in which the differences of the parents are reduced in 

 the offspring to an intermediate blend or average. 



The difference between interpolar inheritance and inter- 

 mediate inheritance is that in the former the intermediate ex- 

 pression of the diverse parental characters is general only in the 

 first or conjugate generation, since the perjugate gametes tend 

 to express the parental characters, as well as the intermediate. 

 But in true intermediate inheritance the gametes also tend to 

 intermediate expression, so that the perjugate generation may 

 be as definitely intermediate as the conjugate. In the language 

 of Mendelism, dominance is lacking, and the germ-cells are not 

 "pure." Both interpolar inheritance and mosaic inheritance 

 will be found, no doubt, to shade off by imperceptible grada- 

 tions into intermediate or completely blended inheritance. Some 

 of the gradations were well shown in Professor Davenport's 

 poultry-hybrids. 



Descent with Specific Diversity {Hyhridisni) . — No absolute 

 distinction can be made between crosses of groups which have 

 become isolated by natural causes and those which have become 

 isloted by man ; but there is usually this ver}' important difference 

 that conscious human selection narrows the network of descent 

 much more suddenly and definitely than unconscious agencies 

 of the environment. Unconscious selection rejects only the end 

 of the procession, but conscious selection saves onl}^ the head, 

 or some other equally restricted section. Species usually do 

 not differ from each other in the same definite ways that narrow- 



^De Vries, H., 1907. Evolution and Mutation. The Monist, 17: 19. 



"The double variety of the corn-marigold {Ch>ysatttkemiim scgcfiim) arose in 

 my garden in a culture in which I was increasing the number of raj'-florets by 

 continuous selection. During four yeais I had succeeded in increasing this 

 number to about sixty on each head, starting from the cultivated variety' with an 

 average of twenty-one. All the ray-florets, however, belonged to tiie outer rows 

 of the heads, as in the original variety. At once a plant arose which produced 

 some^few ligulate florets in the midst of the disc. This indicated the production 

 of a double race. When the seeds of this mutating individual were sown the 

 next year, they yielded a uniformly double group ; and from this time the new 

 variety remained constant." 



