36 



communication with the outside world, netted over so that 

 no insect could reach them from the continent of Europe or 

 elsewhere, many species that are now of frequent occurrence 

 in them would probably become extinct. The geographical 

 position of the British Islands appears to be on the extreme 

 limit of the possible range of many species ; under excep- 

 tionally favourable meteorological conditions a species may 

 establish itself on our shores and produce successive genera- 

 tions extending over several years. There is little doubt 

 that Colias cdnsa, for instance, does exist at times, for several 

 years together, in some specially favoured spots. But there 

 comes a time when conditions are unfavourable, existence 

 becomes insupportable, and the species dies out. 



The migration of birds has for many years past engaged 

 much attention ; a committee appointed by the Royal 

 Society has done good work ; careful watch has been kept at 

 light-houses and light-ships around our coasts and the infor- 

 mation thus obtained carefully recorded. The dates of 

 arrival and departure of known seasonal migrants have been 

 noted, and precise details as to the direction of their spring 

 and autumn movements taken. In this way, and by the 

 systematic labours of a host of independent observers, not 

 only has a vast amount of information been obtained with 

 regard to the movements of known wanderers, but many 

 species not suspected of migratory tendencies have been 

 found to possess them. Even such species as blackbirds, 

 thrushes, rooks, and robins, which we have been accustomed 

 to regard as the most stay-at-home inhabitants of our gardens 

 and hedge-rows, the least given to wandering of all species, 

 have been found to have migratory tendencies strongly 

 developed. So may it be with some insects that we least 

 suspect. 



Although the migration of insects has received some 

 amount of attention at the hands of many of our entomo- 

 logists, as is testified by the numerous records of observations 

 that are scattered through the pages of the various entomo- 

 logical journals, it is small when compared with the 

 systematic work that has been done in regard to bird migra- 

 tion. The possible relation between meteorological conditions 

 and the migratory movements of insects and their ultimate 

 spread also appears to be little understood. 



That migration does take place, and that our insect fauna 

 is materially affected by it, no one in this present day will 

 doubt, but as to the causes of migration, and its exact effect 

 upon our fauna, we have a great deal yet to learn. There is 



