116 



THE AMERICAN NATURALIST [Vol. XLIX 



The proportion of red to pink was found to bear a 

 direct relation to the relative " fertility " of the parents 

 which produced the hybrid. This suggests a causal rela- 

 tion between the two. 



In dealing with "fertility" the difficulty that one en- 

 counters is, that the offspring of any pair may, with 

 respect to this character, differ from either parent, and 

 also differ amongst themselves, forming a graded series 

 running from the most to the least fertile. An individual 

 taken from such a population is an indefinite quantity and 

 will often defeat the purpose of the experiment. In order 

 to simplify this as far as possible, the following experi- 

 ment was planned : 



Four red-eyed, virgin females were each mated to a 

 pink male. Each male was again crossed to several of his 

 own daughters. The records are given in Table XVIII. 

 As a control a number of F x pairs were bred in each case. 

 The records are given in Table XIX. 



A graphic representation of all pairs recorded in 

 Tables VI-XIX, except for the several very unusual 

 ratios, is given in Figs. 1 and 2. 



R0.1 



Fig. 1 contains 99 pairs in each of which the expected 

 ratio was 3:1, with a total population of 82,607. They 

 are distributed as follows : 



