SEASONAL DIMORPHISM OF LEPIDOPTERA. 181 



sists in the fact that the one form seems like a dry leaf on the 

 under side, while the other possesses another marking, and at 

 the same time a number of ocelli. 



Without engaging in the controversy as to the biological value 

 of these ocelli, I do not for a moment doubt but that the colouring 

 with ocelli is also an adaptive one, possibly protective or intimi- 

 dating colouring. If one of the two forms had no biological 

 adaptive significance, it could no longer exist ; the single adaptive 

 one would have replaced it. But it is obvious that the appear- 

 ance of complicated details of marking and colour, such as ocelli 

 are, cannot be simply the direct effect of heat or cold, drought or 

 humidity. These influences are not the actual causes of such forma- 

 tions, but only the stimulus, which sets their primary constituents 

 free, i. e. induces their development, as I tried to demonstrate in 

 the lecture above noted. As the sufficient cause of the winter sleep 

 of the marmots does not lie in the cold, but in the organization 

 of the animal which is adapted to the cold, and as the cold only 

 brings the existing predisposition to winter sleep into play, so 

 among these butterflies with adaptive seasonal dimorphism the 

 display of the one or the other marking is apparently connected, 

 partially at least, with one of the above named outward in- 

 fluences, although in reference to these tropical butterflies we do 

 not yet know to which of them. 



We recognize temperature as the stimulus to development with 

 the cases of adaptive seasonal dimorphism of our indigenous 

 butterflies, as in all cases of seasonal dimorphism, which have 

 been hitherto proved experimentally, it is always high and low 

 temperature, which gives the outward impulse to the appearance 

 of the one or the other form, where this impulse did not come 

 exclusively from ivithin. 



There are therefore two different sources of the appearance of 

 seasonal dimorphism ; on the one hand, the direct action of 

 alternating external influences, viz. temperature, can bring about 

 this change in the outward appearance ; and, on the other hand, 

 the processes of selection. It is therefore necessary to consider 

 these two kinds of seasonal dimorphism separately. It will 

 certainly not always be easy to decide between them when a 

 particular case has to be dealt with, as at present it is not 

 always possible to say whether a colouring or marking has a 

 definite biological value or not. Both causes also may co-operate 

 in one species. 



Direct seasonal dimorphism. — Among our indigenous species 

 Chrysophanus phloeas may be certainly considered an example of 

 this. In the first place, the seasonal differences relate only to 

 the upper side, and then a biological value can scarcely be dis- 

 covered for the black powdering of the summer form, eleus. 

 Moreover, it occurs in both sexes. 



On this point the experiments of Merrifield and my own are 



