Experimental Evolution, Variation, and Heredity 111 



examine this problem. Crosses between Sebrights and Black-breasted Game Ban- 

 tams showed that the suppression of the secondary sexual characters of the 

 plumage of the Sebrights is due to an inherited factor or factors. Castration of 

 the ordinary hen-feathered Sebright male causes the ordinary secondary char- 

 acters to appear in him at once despite the fact that we must suppose that all 

 the other cells of the body contain the inhibitory factor in question. Evidently 

 this factor can cause the production of its specific material only in the testes. 

 Incomplete castration leads to imperfect development of cock-feathers plumage, 

 and as the testes regenerate there is a complete return to hen-feathering. The 

 same kind of results were found for the hen-feathered Fi males and for those 

 F2 birds that are hen-feathered. Despite 'the very great differences in color be- 

 tween the two races the results indicate that three pairs of Mendelian factors may 

 suffice to cover the genetic results. 



An examination of the histology of the testes of the Sebright brought to light 

 the presence of the so-called lutear cells that are absent in the testes of other 

 races of fowls having the secondary plumage characters. Similar-looking inter- 

 stitial cells are known to be present in the hen, and since after removal of her 

 ovary she develops the secondary sexual characters of the cock it is not unlikely 

 that the interstitial cells found in the Sebright cock have the same function as 

 those in the hen and that some internal secretion, produced by both, causes the sup- 

 pression of the sexual type of plumage. 



No. 264. STURTEVANT, A. H. An Analysis of the Effects of Selection. Octavo, 

 64 pages, 1 plate, 14 figs. Published 1918. Price $1.00. 



This volume gives an account of experiments undertaken to obtain critical 

 evidence as to the manner in which selection produces its effects. By taking ad- 

 vantage of the established body of evidence regarding the genetic behavior of 

 Drosophila, and by using known and easily classifiable characters as "markers" for 

 each chromosome, definite proof of the existence of specific modifying genes for 

 bristle number was obtained. The results of selection and of crossing selected 

 stocks were similar to those obtained by most investigators and the demonstrated 

 modifiers are sufficient~to account for the results observed in this case. Evidence 

 is presented indicating that contamination of allelomorphs did not occur in these 

 experiments ; and a critical review of the literature leads to the conclusion that 

 contamination has not been made probable in any of the numerous instances that 

 have been cited as proving its occurrence. Since contamination of allelomorphs 

 has never been demonstrated, but modifying genes have been completely demon- 

 strated in specific cases and will account for all the phenomena in question, the 

 conclusion is reached that selection is ordinarily effective only in sorting out genes 

 already present. 



No. 288. CASTLE, W. E. Studies of Heredity in Rabbits, Rats, and Mice. Octavo, 

 56 pages, 3 plates, 5 text figures. Published 1919. Price $1.50. 



This paper contains a summary of experiments extending over several years, 

 reported upon heretofore only in part. Part I contains the final data and an 

 analysis of the notable effects secured by systematic selection in modifying a 

 Mendelian character, the hooded pattern of rats. These effects are shown by a 

 crucial experiment, to be wholly compatible with stability of the chief gene 

 involved. 



Part II deals with the inheritance of two different forms of white-spotting 

 seen in Dutch rabbits and English rabbits respectively. The one behaves as a 

 recessive in heredity, the other as a dominant. Both are subject to considerable 

 modification through systematic selection, or through crossing with each other. 

 These effects are shown to be due to changes in residual heredity rather than to 

 changes in single genes.' Nevertheless, at least three different forms of white- 

 spotting are shown to occur in Dutch rabbits, each of which behaves as an 

 allelomorph of the others, and further English (though physiologically different in 

 nature from Dutch) behaves as its allelomorph, whether through close linkage 

 or for other reasons is still uncertain. 



