72 Studies in the Theory of Descent. 



Tithonus, Hyperanthus, Ida, drV.), we may admit 

 that during the glacial period such species did not 

 pass the winter as pupae. As the climate grew 

 warmer, and in consequence thereof a second 

 generation became gradually interpolated in many 

 of these monogoneutic species, there would ensue 

 (though by no means necessarily) a disturbance of 

 the winter generation, of such a kind that the 

 pupae, instead of the caterpillars as formerly, 

 would then hibernate. It may, indeed, be easily 

 proved a priori that whenever a disturbance of 

 the winter generation takes place it only does so 

 retrogressively, that is to say species which at one 

 time pass the winter as caterpillars subsequently 

 hibernate in the egg, while those which formerly 

 hibernate as pupae afterwards do so as caterpillars. 

 The interpolation of a summer generation must 

 necessarily delay till further towards the end 

 of summer, the brood about to hibernate ; the 

 remainder of the summer, which serves for the 

 development of the eggs and young caterpillars, 

 may possibly under these conditions be insuffi- 

 cient for pupation, and the species which hiber- 

 nated in the pupal state when it was monogoneutic, 

 may perhaps pass the winter in the larval con- 

 dition after the introduction of the second brood. 

 A disturbance of this kind is conceivable ; but it is 

 certain that many species suffer no further altera- 

 tion in their development than that of becoming 

 digoneutic from monogoneutic. This follows 



