APPEARANCE OF BUTTERFLIES 255 



not appear that our butterflies suffer particularly 

 from an exceptionally cold or long winter, but 

 rather from unusual warmth, sufficient to arouse 

 insects from torpor at times when hibernation 

 should be expected ; or, in the fair season, di- 

 rectly from long continued storms and moisture. 



The fluctuation therefore in the numbers of our 

 butterflies is probably due in large measure to the 

 activity or inactivity, the abundance or rarity, of 

 their active enemies, and especially, considering 

 how extensive their depredations, to the abun- 

 dance or otherwise of their parasites. It is the 

 strikino- of the balance which exists between a 

 creature and its enemies in the struggle of each 

 for its own existence. Let some event, untoward 

 to it, decrease the ratio of the parasite, — the 

 butterfly flourishes ; but its very consequent su- 

 perabundance the following year only gives a bet- 

 ter pasturing ground to the parasite, reduces the 

 butterfly below the normal, and causes the para- 

 site to abound inordinately, only to find its food 

 supply cut off by its own voracity and inconti- 

 nence and the scales again to be turned. It is 

 then this perpetual warfare, this unending, inex- 

 orable struggle for existence, testing the fitness to 

 survive, which is the prime cause of periodicity in 

 the abundance of a given species. 



