168 THE AMERICAN NATURALIST [Vol. XLVII 



Theoretical. — Certain of my breeding experiments have 

 seemed to show that the Brown Leghorn plumage colors 

 and pattern followed a sex-limited mode of descent. Cer- 

 tain difficulties, however, have arisen. As stated above, 

 it has become evident that the coloration and pattern of 

 the Brown Leghorns is not due to a single inheritable 

 unit, but to a complex. This means that its more or less 

 independent parts must be discovered and the mode of 

 inheritance as well as the conditions under which each 

 part becomes visible in the soma, worked out. Until this 

 is done it will be impossible to say what part sex-limited 

 inheritance plays in determining sexual differences. The 

 following representation, then, is merely an attempt to 

 formulate a working hypothesis. 



The male may be considered to be duplex for an internal 

 secretion 8, produced by the testes, which is necessary 

 for the full development of the comb and less clearly for 

 the crowing instinct and sexual behavior, but not for 

 plumage and spurs. In the female this secretion is re- 

 placed by one S' produced by the ovary. (S' may, per- 

 haps, stand in some simple chemical relation to S.) To 

 its effect is to be referred the female's form and color. 

 When the ovary is removed or becomes pathological so 

 that its normal secretion is no longer produced, then the 

 male characters develop to an extent which is determined 

 by factors at present unknown to us. Nor are we con- 

 cerned here as to what part of the ovary or testis pro- 

 duces this secretion (or secretions, for S and S', respect- 

 ively may well represent a number of secretions which 

 behave in the same general way). In any case the in- 

 ternal secretion may be conceived to be associated with 

 sex in the same manner as any other sex-limited char- 

 acter. The formula for the male would then be S<$ SJ y 

 that for the female S' is of course to be consid- 



ered dominant to S. If this scheme represents the actual 

 condition of affairs it may not be necessary to suppose 

 that the female Brown Leghorn is simplex for L where L 

 represents the whole or a part of the male Leghorn 



