12 



yellow ground coloration of the hindwing underside characteristic of 

 the wet season, while the dark veining of the same area, a feature 

 belonging to the dry-season phase, was not only present but was 

 strongly accentuated. 



There is still another set of facts to be taken into account before 

 we can claim to have exhausted even the elements of Seasonal 

 Dimorphism. It is this: that even in cases where the dependence 

 upon season of the change in aspect is well established, the two 

 forms may often be found on the wing at the same time. Mr. 

 Crawshay captured all three forms of Precis sesamns, the " wet," the 

 "dry" and the "intermediate," within little more than a week of 

 each other. The two forms, or phases, of Catopdlia poiiwua, were 

 observed together in large numbers in the Kangra Valley of the 

 Western Himalayas by Mr. G. C. Dudgeon. Other naturalists have 

 recorded similar facts both of Catopsilia pomona and of its near 

 relative Catopsilia pyranthe. And yet there is no doubt, on the 

 testimony of quite first-rate authorities, that at all events in many 

 parts of their range the diverse phases of both these butterflies are 

 to some extent dependent on the change of season. Many other 

 instances could be given of the simultaneous occurrence of forms 

 well kno'ftn to be seasonal, and also of forms which are closely 

 analogous to ascertained cases of seasonal dimorphism, though the 

 actual proof of their seasonal character may be lacking. Some of 

 these instances of simultaneous occurrence may be put down to 

 overlapping at the change from wet to dry and vice versa, the new 

 brood emerging while the former one is still on the wing. But 

 there are many cases which cannot be explained in this way, and 

 indeed all that we can safely say seems to be that several of these 

 dimorphic phases, though under certain circumstances and in 

 certain parts of their geographical range, they may be confined to 

 their respective seasons, under other circumstances and in other 

 localities may occur indiscriminately and without reference to the 

 meteorological conditions prevailing at the time of their appearance- 

 Taking the fact of these irregularities into consideration, we find it 

 by no means easy to decide between two opposite views of the origin 

 of seasonal dimorphism. On the one hand the dimorphism may be 

 supposed to have arisen as a direct adaptation to external conditions, 

 and afterwards, in consequence of extension of the range of the 

 species, or of alteration in those external conditions, to have more 

 or less fallen out of correspondence with the changes of season. On 

 the other band the dimorphism may have had an entirely indepen- 

 dent origin, but have subsequently been brought, under the influence 

 of natural selection, into a more or less intimate relation with local 

 and seasonal conditions. It is quite conceivable that both principles 

 may have been concerned in the production of the state of things 

 now open to our observation. 



Two further facts may be borne in mind in considering the 

 simultaneous occurrence of different seasonal forms. One is that 



