SEASONAL DIMORPHISM OF LEPIDOPTERA. 245 



ancestral form on the concurrence of exceptionally favourable 

 circumstances ; it is a normal change between two germ-determi- 

 nants, one of which is without doubt the older. 



The origin of this alternation is not difficult to explain. With 

 species, which, like V. levana and P. napi, were single-brooded 

 during the glacial epoch, and were then subject to similar con- 

 ditions of life as our present spring form, some of the ids of the 

 germ-plasm, which in earlier times were homogeneous, have been 

 gradually formed into summer ids, and at the same time the 

 activity of these ids has been controlled by certain internal and 

 external influences, which have resulted in the constant alterna- 

 tion of the two forms. The point is, that each of the two 

 adaptive forms should arise at the right time, but not at a time 

 and under external conditions, to which the protective colouring 

 is not adapted. The facts teach us that this happened in a two- 

 fold way : First, from the fact that the two kinds of ids are so 

 arranged, that they alternately promote the ontogeny ; in the 

 first brood therefore the winter ids would be active, in the second 

 brood the summer ids, in the third brood the winter ids again, 

 &c. And secondly, from the fact that the activity of that kind 

 of id, which is disposed to be active, can be hindered by certain 

 external influences, heat or cold, and at the same time the oppo- 

 site kind of ids can be induced to become active. It comes to 

 this, that the internal disposition to activity, of whose nature we 

 can obviously divine nothing, is not always connected with an 

 alternation of the two kinds of ids, but that in a varying per- 

 centage of individuals of any brood the same ids are also disposed 

 to be active in the following brood. But in this case, as a rule 

 at least, the determination of the winter id is connected with the 

 tendency to latency (hybernation), the determination of the 

 summer id with the tendency to immediate development. 



In this way the coincidence of both adaptive forms with the 

 'conditions of life appertaining to each seems to be best assured. 

 Generally only two broods fly with us, and for this therefore the 

 regular alternation of the two forms from internal causes is 

 sufficient. Now it still remains to meet the possibility that by 

 very unfavourable influences (bad weather, preponderance of 

 enemies), such as constantly recur from time to time, though, 

 indeed, but rarely, the permanence of the species in a certain 

 area might be destroyed ; and this happens here, as with many 

 other species, from the fact that a percentage, varying in each 

 brood, carries with it the predisposition to delayed development. 

 But in the normal way this is connected with the disposition to 

 activity on the part of the winter ids, since otherwise the summer 

 adaptation might arise in the spring. In each brood, however, 

 there are also individuals whose summer ids are disposed to be 

 active, but these generally possess also the predisposition to 

 immediate development ; this applies to all specimens, which 



