( lxxiv=§) 
complete resistance to altered temperature, so frequent in 
the experiments of Weismann and others, point very clearly 
to the operation of some other factor than the degree of 
humidity, or of temperature ; but it must be admitted that 
we are as yet quite in obscurity as to its actual nature, and 
that our investigations into seasonal dimorphism must be far 
more systematically and thoroughly prosecuted before con- 
clusions of a satisfactory character can be arrived at. 
While the observations already on record, to which I 
have drawn attention above, render it beyond question that 
seasonal dimorphism is of world-wide prevalence, it is at the 
same time surprising—considering the great and increasing 
study devoted to exotic butterflies of late years—that so very 
little is definitely known of the actual range and conditions of 
its occurrence beyond European limits. So far as the Pale- 
arctic Region is concerned we are indebted to Standfuss* for 
a comprehensive list of the cases recognised, distinguish- 
ing between those where the seasonal disparity 1s so marked 
as to have led to the bestowal of distinct names on the 
two forms, and those where the disparity is less and no 
second name has been given. In the former category there 
are 23 cases (17 in Butterflies and 6 in Moths), and in the 
latter 15 (14 in Butterflies and 1 in Moths), making in all 38 
cases, viz., 31 in Butterflies and 7 in Moths. The butter- 
flies comprised in the more marked category include | case 
in Satyrine, 2 cases in Nymphaline, 5 in Lycenide, 6 in 
Pierine, and 3 in Papilionine; while those in the less 
marked category are three cases in Satyrine, 3 in 
Nymphaline, 3 in Lycenide, and 5 in Pierine, so that 
taking the totals of both categories in their order of 
numerical importance we have 11 cases in Pierinew, 8 in 
Lycenide, 5 in Nymphaline, 4 in Satyrine, and 3 in Papilion- 
ine. The moths are all ranked in the more marked category 
with the exception of a Liparid (Dasychira abietis); they 
are two in the Bombycide and four Geometers. The number 
of known cases in the Palearctic Region thus appears to be very 
* © Handbuch der Paliarktischen Gross-Schmetterlinge fiir Forscher 
und Sammler,” Ed. 1896, p. 229. 
