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PROCEEDINGS OF THE AMERICAN ACADEMY. 



the group with the exceptional structure. Other contributions to this 

 law from other groups of animals are needed. 



The second question bears upon the significance of sexual dimorphism. 

 The quantitative studies of Davenport and Bullard('9Gj upon the glands 

 in the fore legs of pigs show the variability of the males to exceed that 

 of the females by 2.5% ; while, on the contrary, Pearson ('97) shows that 

 women are in general slightly more variable than men, — exhibiting the 

 greater degree of variability in eleven out of a total of seventeen char- 

 acters under consideration. 



II. Method. 



The species upon which I have worked is the moth Thyreus ahhotii 

 Swainson, one of the common Sphingidne of Eastern North America. It 

 differs from its allies most notably in that the outer margins of the prima- 

 ries are excessively irregular and extraordinarily long, as compared with 

 other dimensions of the wings. I have tried to find an answer to the 

 question. Is this extraordinary outer margin more variable than other 

 more ordinary characters of the same wing? 



a — 



THYREUS ABBOTII SWAINSON : Right Primary. 

 {Enlarged Shiidow.) 



Figure 1. 



For measurement and comparison with the margin (1), I selected 

 the following three dimensions for measurement: — 



(2) The length, from insertion to apex (a-h in Figure 1) ; 



(3) The greatest breadth, — the perpendicular distance of the poste- 

 rior protuberance of the outer third of the wing from the costa {c-d) ; 



(4) The length of an imaginary straight line drawn between the 

 extremities of the sinuous margin, — that is, from apex to posterior 

 protuberance (h-d). 



