ON THE PROFUSION OF RHOPALOCERA IN THE ALPS. 289 



ing about 20 different species, which gives an average of UV50 

 apiece. Tliis occurred at an elevation of some 800U feet above 

 sea-level, in the upper alpine zone above the belt of pine trees. 

 At Kandersteg, some 4000 feet lower, among the conifers, I netted 

 200 specimens one morning, representing about 85 species, which 

 gives an average of about 6 apiece. Both events happened in the 

 month of July, and may be taken as fairly typical of the relative 

 proportions of specimens to species which can be obtained in the 

 regions in question in comparison with their corresponding 

 numbers in the tropical zones. The largest number of species 

 which I have met with in a single day in the Alps is about 40 in 

 the month of July, but the average appears to be considerably 

 less, being somewhere about 30 for the lower and between 10 and 

 20 for the upper alpine region. In the South of England the 

 three largest hauls which I have ever made occurred in the 

 months of August, May, and July, upon which occasions I 

 captured 21, 20, and 18 different species respectively; but the 

 average generally falls far below these figures, even in the best 

 localities. The percentage of specimens to species appears to 

 approximate very closely to that occurring in the Alps in 

 equivalent climatal zones. 



I will now consider the causes which have produced the fore- 

 going phenomena. In conformity with the theory of evolution by 

 means of natural selection, it may be taken for granted that 

 whenever a species is scarce its struggle for existence is very 

 great; on the other hand, when it is plentiful it has not many 

 enemies to contend with. In the tropics butterflies have many 

 enemies to contend with in the shape of insectivorous birds, 

 reptiles, batrachians, bats, dragonflies, spiders, besides a host of 

 other parasites. Competition between closely-allied species must 

 also be exceedingly great. These must all be providing plenty of 

 scope for the play of natural selection, and thus new species will 

 constantly be given ample opportunity of coming into existence. 

 On tlie other hand, adverse climatal influences will have little 

 effect in curtailing their numbers. Unlike the temperate regions 

 of the earth, the tropics have, so far as we know, never experienced 

 the ill effects of a glacial period, whicli has proved such a powerful 

 agent in reducing the number of species which have from time to 

 time established themselves within the sphere of its periodic 

 influence. Species in the tropics have therefore been permitted 

 to accumulate without any restraint of this nature for many 

 thousands of years. 



In the Alps, on the contrary, the number of enemies which 

 prey upon butterflies is comparatively insignificant. In the sur- 

 rounding plains reptiles, batrachians, and birds are pretty plenti- 

 ful ; but it is precisely here where the greatest number of species of 

 Rhopalocera occur, which is in direct confirmation of my theory; 

 while among the mountains, above a certain elevation, they are 



