( lvii ) 
12°—24° R.) immediately after pupation, pupe from eggs laid 
by the August brood of the summer form, prorsa ; but here 
the artificial temperature had little or no effect, all, or nearly 
all, the pupe hibernating, and emerging in the following 
spring as the pure winter form /evana. This latter result led 
the author to the opinion that cold and warmth could not be 
the immediate causes of a pupa emerging in the prorsa or 
levana form ; and that the explanation of the facts seemed to 
be (a) that the winter form /evana is the original type of the 
species, seeing that it was found possible to make many 
specimens of the summer form prorsa revert to it by means of 
cold, whereas the converse change could not be effected; and 
(6) that the species originally existed in the glacial period as 
a single-brooded and monomorphic butterfly, and only became 
double-brooded and gradually developed the prorsa form as 
warmth of climate increased. 
With Pieris napi, Weismann found the pupe from eggs 
laid by the winter form much more responsive to the action 
of cold (applied immediately after pupation and continued for 
three months) than those of A. levana, by far the larger 
number emerging as the pure winter form when transferred 
to a hot-house, and the remainder (which resisted forcing and 
hibernated) all producing the same form in the following 
spring. The converse experiment was not tried with the 
pupe of ordinary P. napi, but with those of the Alpine and 
Polar variety, bryonia, but the result was in accordance with 
that of the corresponding experiment in the case of A. levana 
—the application of heat had no transforming effect, and all 
the butterflies emerged as pure bryoniw. Weismann was thus 
led to regard the single-brooded variety bryonie as the original 
form of the species from the glacial period, and napi in its 
winter and summer forms as gradually produced under 
increasing climatic warmth. 
The experiments conducted with so much skill and perse- 
verance by W. H. Edwards with the North-American 
Papilio ajax and Phyciodes tharos yielded much the same 
results as those obtained by Weismann in Europe. In the 
complicated case of P. ajax—where the winter form presents 
itself in the two differing generations known as walshii and 
