PERIODICITY IN OKCANIC LIFE. 59 



migrations and the abundance of that animal. I might cite ahiiost 

 any number of instances of periodicity in insects, but I will only 

 refer to a few more cases, taking the well-known ones. 



The Black-veined \Miite Butterfly (Aporia crahegi) used to be 

 common in many parts of England. But where is it now ? It is 

 getting rarer every year in the few localities where it is still found, and 

 looking as if it would soon become extinct. Can any explanation be 

 given of its diminution ? I know of none. The same may be said 

 of the Great Blue Butterfly (Lyaena arionjda^^ many others. Again, 

 numerous species which the earlier Aurelians considered rare are 

 now common. Why is this ? I may be told we know better how 

 to find them, but I question if this is the true explanation. I should 

 rather say the species is undergoing the condition of periodic abun- 

 dance, quite independent of any proceedings of man. In many 

 species we, who have given years to the study of insects, know it is 

 not improved knowledge or ability that enables us to find more of 

 them, and I would use the same reasoning in dealing with the views 

 of the older Aurelians, and say that it is not greater skill, but 

 rather that the insect is more abundant. 



There is one other insect I should like to refer to in illustration 

 of periodicity. In this (the Colchester) district, in the third and 

 fourth decades of this century, the Blister-fly (Lytta vesicatoria) was 

 very abundant on every ash tree ; at the present time it is rare. 

 Every variety of weather occurred during these years, and the 

 variations from hot to cold and from wet to dry, have been as 

 frequent since ; and therefore is it not reasonable to suppose that 

 meteorological conditions have had no effect ? Man's proceedings in 

 this case certainly had none ; for the number of beetle collectors 

 were so few, and none of these insects were captured for commercial 

 purposes ; therefore some other cause must have been at work, and 

 what was it ? It could not be natural enemies as birds, for the birds 

 leave them alone, neither could it be want of food, for as far as we 

 can see all their various articles of diet were equally common 

 throughout both periods. 



No attempt has ever been made to explain that scarcity or 

 abundance of Fish, which is so marked in the gregarious species, 

 such as cod, herring, sprat, mackerel, etc., excepting that it is said 

 " It is a bad season," or that they have left the part of the coast 

 where they previously abounded. This change of habitat, no doubt, 

 explains the scarcity in certain localities, but something more than 



