( xxyii ) 
cealed by its harmony with the yellow winter foliage, while 
that of natalensis is dull brownish-black, with black patches 
on the wings, and is protected among the dark withered leaves 
of the moist summer. In one case he has bred the red form 
from a gilded pupa, but never sesamus froma dark one. It 
would be of great interest to ascertain whether these 
pup exhibit true seasonal differences, or whether, as seems 
more probable, the mature larve are susceptible to the 
colours of their immediate environment, as has _ been 
proved to be the case with those of several of the allied 
Vanessidee. 
“Of still more importance is the decision as to the cause 
or causes of the seasonal differences of the imago. On this 
question Mr. Marshall maintains that ‘it at once becomes 
evident that the directly exciting cause is a climatic one’ 
(/.c., p. 37), and he argues that moisture, rather than tempera- 
ture, supplies the efficient stimulus. The great want in the 
matter is experimental inquiry on as extensive a scale as 
possible, carried on over a large part of the year, so as to in- 
clude larvee and pupe, that in the ordinary course certainly 
produce the blue form, as well as those that produce the red, 
and those that, at intermediate times, are known to produce 
both forms. 
“ Concerning those last-mentioned, Mr. Marshall has shown 
conclusively that the offspring of a single parent, brought up 
under the same conditions, may belong to both forms; in 
fact, in his experiment already described, the summer form 
emerged five days later (¢.e., when the winter season was to 
that extent advanced) than the winter form. This fact leads 
us to contemplate the possibility that the change of form is 
due to internal causes and independent of, although on the 
whole synchronous with, the seasons. 
«‘ And even if some exciting cause be proved to be necessary, 
it is probable there are great differences in the internally 
caused degree of susceptibility to stimulus in the two seasons, 
while a mixed susceptibility exists in the intermediate 
period, 
“Tt is likely that Mr. Marshall’s observations are to be 
explained by one or other of these alternatives.” 
