io Studies in the Theory of Descent. 



and duration of development, i. e., duration of the 

 chrysalis period. The duration of the larval 

 period need not engage our attention, as it is only 

 very little shorter in the winter brood at least, it 

 was so with the species employed in the ex- 

 periments. 



Starting from these two points of view, I carried 

 on experiments for a number of years, in order to 

 find out whether the dual form of the species in 

 question could be traced back to the direct action 

 of the influences mentioned. 



The first experiments were made with Araschnia 

 Levana. From the eggs of the winter generation, 

 which had emerged as butterflies in April, I bred 

 caterpillars, and immediately after pupation placed 

 them in a refrigerator, the temperature of the air 

 of which was 8 IO Q R. It appeared, however, 

 that the development could not thus be retarded 

 to any desired period by such a small diminution 

 of temperature, for, when the box was taken out 

 of the refrigerator after thirty- four days, all the 

 butterflies, about forty in number, had emerged, 

 many being dead, and others still living. The 

 experiment was so far successful that, instead of 

 the Prorsa form which might have been expected 

 under ordinary circumstances, most of the butter- 

 flies emerged as the so-called Porima (Figs. 3, 4, 

 7, 8, and 9, Plate I.) ; that is to say, in a form inter- 

 mediate between Prorsa and Levana sometimes 

 found in nature, and possessing more or less the 



