384 castle: role of selection in evolution 



in later generations show either no segregation or imperfect 

 segregation (fowls, Pearl 2 ). I do not say that in these cases no 

 Mendelian inheritance is involved, but merely that no stable 

 genes are in evidence, nothing that would preclude the probable 

 effective use of selection in maintaining or raising breed standards. 



If we turn from the breeding of animals, in which manifestly 

 the pure line principle has little applicability, to the breeding 

 of plants other than those which are self-fertilized, we again 

 find that this principle has a very limited applicability. Prob- 

 ably the most valuable open pollinated field crop in cultivation 

 is maize. But a pure line of maize is not known to exist. An 

 experiment which should have lead to the production of pure 

 lines, if such a thing were attainable in maize, has been in prog- 

 ress at the University of Illinois for the past twenty years. 

 Selection has been made for increased and for decreased protein 

 content of the grain, and also for both increased and decreased 

 oil-content, with the result that steady progress in the direction 

 of selection has in every case been made. The high protein 

 strain now contains twice as much protein as the low protein 

 strain; and the high oil strain contains four times as much oil 

 as the low oil strain. The divergence of the selected lines from 

 each other is not now as rapid as at first but it still continues 

 steadily, with no indication that it is soon to cease, as must be 

 the case if only stable genes were involved. 



Those characters in maize which directly affect the yield, 

 such as size of plant, or of the grain which it bears, are blending 

 in inheritance and show imperfect segregation subsequently. 

 They are probably all of them quite as amenable to selection as 

 the oil content and protein content of the seed, experimented 

 upon in Illinois. 



2 It is true that Pearl (1912) has described fecundity in fowls as "typically 

 Mendelian" in heredity but his figures show that in crosses between Barred 

 Rocks and Cornish Indian Games, the average fecundity of the Fi birds is in 

 both the reciprocal crosses intermediate between that of the respective parent 

 races though nearer the racial average of the sire, which supports his contention 

 that a sex-linked gene is involved, but shows also that this is not the only factor 

 involved. Back-crosses of F x of both sexes with the pure races give evidence of 

 further blending (or imperfect segregation) on the part of the non-sex-linked 

 factor or factors. 



