14 MUTATIVE REVERSIONS IN COTTON. 



REVERSIONS OF LATER GENERATIONS OF HYBRIDS. 



Many attempts have been made to obtain early and prolific Eg^'p- 

 tian or Sea Island varieties bv crossino; with Upland, and the first 

 generations (^f such crosses often appear very promising. The dilli- 

 culty is that the later generations not only revert to the parental 

 types, but often go farther back, to the condition of remote unim- 

 proved ancestors. Instead of having longer lint than the Egyptian 

 parent, as the first generation usually does, the later generations 

 become inferior even to the Upland parent. Hybrids representing 

 the fourth and fifth generations, grown at San Antonio, Tex., in the 

 season of 1909, did not show a single plant with good Eg3'ptian lint, 

 and very few that were better than ordinary Upland. This extreme 

 deterioration might be ascribed partly to adverse conditions, but first- 

 generation hybrids grown under the same conditions produced excel- 

 lent lint, longer and stronger than the Egyptian parent. These con- 

 trasts between the different generations show that the hybrids do 

 not merely fail to fix particular combinations of the parental charac- 

 ters, but may first exceed the parents and then suffer serious deteriora- 

 tion. The characters of the lint that have received the most selec- 

 tion show the most striking deterioration. Such hybrids promise to 

 have practical value only in the first generations. The problem of 

 utilization turns upon the possibility of raising commercial quantities 

 of hybrid seed." 



The fact that hybrids of later generations often show charactere 

 different from those of the first generation has been taken as proof 

 of the Mendelian theory of separate transmission of contrasted char- 

 acters. Characters that appear in all of the individuals of the first 

 generation l)ut not in all of the secontl or later generations have been 

 ascribed to the presence of two independent Mendelian "factors" 

 that are supposed to be transmitted separately, and not recombined 

 in all the members of the later generations, but in only half of them. 



This theory would explain why half of the second generation might 

 fail to show a character that appeared in all of the first generation, 

 but it gives us no suggestion of the complete disappearance of the 

 long lint in the later generations of the cotton hybrids. There is no 

 reason to suppose that the internal "factors" that produce the long 

 lint in the first generation of a hybrid cease to be transmitted to the 

 later generations, but there are serious differences in the external 

 expression of the characters. Factors that influence the expression 

 of characters have to be considered, not merely the possibilities of 

 alternative transmission. A character that has been expressed in 



" Snppressod and Intensified Characters in Cotton Hybrids. Bulletin 147, Bureau 

 of Plant Industry, U. S. Dept. of Agriculture, 1909, p. 15. 

 [Cir. 53] 



