206 



bulletin: museum of comparative zoology. 



1. Relative Infertility of Certain Combinations of Gametes. 



Tutt ('98) reports that in crosses between two nearly related species 

 of Lepidoptcra, Tcphrosia bistorta and T. crcpuscularia, it has been 

 found that when l)istorta is the male parent, the hybrid offspring show 

 a normal distribution as to sex, a slight excess of males. See crosses 

 [1] and [2] in Table I. r)at in the reciprocal cross, with crepuscularia 

 (or its dark aberration, delamerensis) as the male parent, the olfspring 

 are practically all males. See Table I., crosses [3] and [4]. 



TABLE L 



Sex-proportions among hco gcnerntionx of hybrid offsprinrj of Tephrosia lmtorta{V>) 

 X T. crepusndaria (C) or the dark aberration of the latter, delamerensis (D). 

 [Statistics of Tutt ('98).] 



a « 



2i-* 

 t-> . 



x> a 

 >.« 

 WO 



Wo 



Hybrid female offspring of bistorta ^ X delamerensis 9 (cross [2], 

 Table I.) when crossed with crepuscularia ^ gave (cross [6], Table I.) 

 a large excess of males, as we should expect on the Mendelian hypothesis 

 that tlie hybrid furnishes in equal numbers gametes haviug the pure 

 character of either parent race. For we should exj)ect the combination 

 of pure delamerensis with crepuscularia gametes, wliich would occur in 

 half the total cases, to yield offspring having the normal sex-proportion, 

 a slight excess of males (compare cross [1],, Table I.) ; but pure bistorta 

 ova fertilized by crepuscularia sperm should yield only male offspring 

 (compare cross [3], Table 1.). Accordingly the result to be expected is 

 3+^:19; the observed result is 38 ^J : 11 9 . 



To explain the peculiar sex-distribution observed in these crosses, we 

 may make two simple hypotheses, which, I believe, are warranted by 

 the facts observed. (1) 7'he sex-character borne by a bistorta (B) gamete 



