9 



are not so much those which pursue and capture hutterflies on the 

 wing, as those that track them down and seize them when settled 

 and at rest. 



From all this it appears that we can with a good deal of proba- 

 bility attribute a significance founded on utility to many of these 

 instances of seasonal dimorphism. But, as so often happens in 

 biological enquiry, before we have gone very far we have to recog- 

 nise that the case is not so simple as it looked at first sight. Let 

 us consider another instance. The little golden-red copper butterfly, 

 so familiar an object in our fields and gardens, has several broods in 

 the course of the year. In a certain part of its range the butterfly 

 is seasonally dimorphic, the spring brood being of the well-known 

 golden-red colour, and the subsequent brood or broods, which have 

 passed through all their stages in the late spring and summer, 

 showing an intermixture of dark scales which give a difi'erent aspect 

 to the perfect insect. It is impossible to maintain with any confi- 

 dence that this change in appearance is of advantage to the species. 

 We must be content with the knowledge that the dimorphism is in 

 some way or other dependent on climatic conditions, but at this 

 point we have to stop. We cannot, with our present knowledge at 

 any rate, claim the seasonal variation of the copper butterfly as an 

 adaptation to the varying conditions of its environment ; the alter- 

 nation between light and dark must be put down as the direct result 

 of some external influence, probably that of temperature. The same 

 is very likely the case with other instances of seasonal dimorphism, 

 those for example which occur in the white butterflies belonging to 

 the African and Asiatic genus Belenois. 



But whether the seasonal changes are adaptive, that is to say 

 beneficial, or not, there is no doubt that they are determined by the 

 action of some external condition upon an organism which is so 

 constituted as to respond to that action in a particular direction. It 

 is not a case of simple alternation between the two phases, because 

 we find in many instances that the number of successive broods 

 belonging to one phase depends upon the character of the particular 

 season to which that phase belongs. In tiopical countries, for 

 example, it is often the rule that more generations of a given species 

 are produced during the rains than during the drought. An excep- 

 tionally prolonged wet season will give rise to an exceptionally large 

 number of successive broods, all of the wet-season form. These wet- 

 season broods will go on being produced until the weather changes, 

 when they will in due course be replaced by the dry-season phase. 

 If the species were so constituted as to produce say three wet-season 

 generations to one dry in regular succession, the change ni the but- 

 terfly would simply follow the almanack without reference to any 

 hastening or retarding of the change of season ; but, as we have 

 seen, the generations of the butterfly tend to be regulated by the 

 weather, however much this may happen to be out of correspondence 

 with the usual conditions prevailing at the actual time of year. 



