28 
PAPERS READ BEFORE THE SOCIETY. 
NOTES ON THE EFFECT OF CLIMATIC CONDITIONS ON SEXUAL 
DIMORPHISM. 
(Read January 21st, 1908, by Dr. G. G. C. HODGSON.) 
As a contribution to this discussion, the facts which one brings are 
few, with a context most imperfect, from an experience very fragmen¬ 
tary. One finds, too, that the subject if not new, is one which has at 
most attracted but little attention ; one therefore introduces the subject 
for discussion in the hope that it may reach, so far as a decision to 
night, that it is worth further observation, and not in the expectation 
of a decision in favour of the views expressed in these notes. 
The years with which these remarks deal are principally 1887, 
1888, 1889, 1900, and 1907, with less reference to 1890 and 1897, 
1908, 1£04, 1905, and 1906, and the species mainly Argynnis aylaia, 
Arr/iades bellary us and corydon, and Polyammatus icants. 
In altered sexual dimorphism: — 
i. For increase, alteration of one sex alone is necessary, but is often 
the result of diverging variation in both sexes in various directions (by 
either disappearance or exaggeration of usual characters, or appearance 
of unusual), i.e., by “ difterentiative” changes. 
ii. For decrease of sexual dimorphism changes not necessarily 
required in both sexes. When in both sexes may be a “ rectilinear” 
or direct approximating tendency, or by “ diversion ” from normal 
line to a common goal, a more subtle form, or in a still more subtle 
form is the result of a more considerable change in one sex, the 
further from the common goal towards which both are tending, by 
which the further tends to overtake or overtakes that which was the 
nearer, in average form, by “sequence.” 
The proposition is, that in some species of Rhopalocera (in this 
country) conditions of climate and allied influences, when markedly 
abnormal, tend to produce effects in the colours and markings of the 
g s or 5 s (or both) such, that the sexual dimorphism is on the whole 
altered, increased or diminished, i.e., to take, e.y., the summer season 
for especial points in the two chief varieties of abnormal conditions 
(the proposition will run):— 
i. Summer seasons with excess of summer conditions tend to 
increase the sexual dimorphism of certain butterflies. These may be 
termed “superaestival” seasons (probably include as the most important 
influences those akin to and those labelled active, but as well as fine 
weather, sunshine, heat, include possibly all favourable conditions to 
full extent, herlthy food also). 
xviii. 
