52 PERIODICITY IN ORGANIC LIFE. 



of definite duration however, when the individuals of a species are 

 very abundant or the reverse, scarce and apparently extinct, or nearly 

 so. The scarcity or abundance in any year of insect life would 

 appear to be quite independent of seasons, or of food-supply, or of 

 any other surroundings, as far as our present knowledge extends. 

 True, there are occasionally years of great heat, and therefore of 

 presumably favourable effect on animal life, but it is not to these 

 years and their effects that I wish to draw the attention of naturalists ; 

 the explanation of the abundance of life in such periods being so 

 obvious to all. It is rather to the fact that there are periods in the 

 existence of every species, when from no apparent cause the indivi- 

 duals gradually increase year by year until we have them in great 

 abundance. 



After this period of what may be termed prosperity of a species, 

 which may continue for some years, a decline begins, when the 

 numbers gradually go down until it would appear that the species 

 becomes nearly extinct. This period of scarcity may also be of 

 some long continuance, or it may possibly lead to the extinction of 

 the species in reality. The extinction of any species of insect by 

 artificial causes very rarely happens, but is much more common 

 amongst mammals and birds, especially with those creatures having 

 a. limited habitat. The case most frequently quoted is that of the 

 Dodo, but there are many others equally decisive, and as clearly due 

 to man's interference with Nature; such as the Rhytina in Behring's 

 Island, the Moa in New Zealand, and the last, and the most dis- 

 graceful case of wanton destruction, that of the Bison in North 

 America, and many others. 



One insect is said to have been destroyed during the last few 

 years by the hand of man. I refer to the Great Copper Butterfly 

 {Po/yommatus dispar). It is said that the drainage of Whittlesea 

 Mere, by destroying the plant on which the caterpillars fed, caused 

 the extermination of the butterfly. This may have been so ; i)ut I 

 very much doubt the assertion. The habitat certainly was not con- 

 fined to Whittlesea Mere, and even if it were, the food-plant (the 

 (Ireat Water Dock, Riiinex hydrolapatJiuni) abounded everywhere in 

 the district where sufficient water was present. On this it might 

 have remained, perhaps in greatly diminished numbers, and it may 

 possibly now be still existing. Although the grand butterfly has not 

 for some years gladdened the eyes of the collector, I should never 

 be surprised to hear that it had been again discovered in some 



I 



