May, 1917.1 \ lVlr\' X t^ i ^ y gy 



It is always difficult, and — in tli^'jiyjfii^f^^eo^ii^ff knowledge 

 — generally impossible, to give a conclusive explanation of any particu- 

 lar ease, depending as it does, on specific bionomics of which we are 

 probably ignorant, on weather conditions reacting in an obscure 

 manner during probably a long period, and on an extremely complicated 

 inter-relation and correlation of any given species with many other or- 

 ganisms. Nevertheless, the main outlines of the general determining 

 causes are clear enough. The exceptional scarcity of species in any 

 given year may be due to : — 



(1) Unfavourable weather conditions, reacting directly on the 

 organism during some stage of its life-history, e.g., exceptionally severe 

 or mild and open winters ; dry or wet svimmers ; an early spring with 

 late frosts ; heavy rains or floods during some critical period of its 

 life, etc. 



(2) Weather conditions reacting indirectly by affecting in an 

 unfavourable manner its food supply. 



(3) Weather conditions reacting indirectly hy affecting its 

 enemies in a favourable manner ; owing to this, there will be an in- 

 crease in the numbers of its parasites or predatory enemies, or in the 

 number of organisms that share with it its food supply. Thus it is a 

 generally observed fact that many insects are rare after a very open 

 winter, partly owing to the fact that they are less torpid and therefoi'e 

 are more likely to be exposed to dangers from frosts, floods and 

 birds ; and partly because the birds have a better chance of search- 

 ing for insect food during mild weather than when there is much 

 snow and frost. Again, if the early-feeding Lepidopterous larvae, 

 notably such as feed on oak, are especially numerous, they sti'ip the 

 trees of their foliage, and the late-feeding larvae either die in great 

 numbers owing to scarcity of food ; or — an interesting modification of 

 habit — in some cases, and to some extent at least, adopt a cai-nivorous 

 instead of a vegetarian diet. 



It is obviously impossible, in the limits of a short paper like this, 

 to discuss in anything like an adequate manner, any of these factors 

 which tend towards the rarity or localisation of animal and vegetable 

 forms, but perhaps enough has been said to show that the whole sub- 

 ject is an extremely complicated one, involving, as it does, a great 

 number of factors — physiological, morphological, geological, physio- 

 graphical, climatic, etc. — concerning whose exact facts and laws, to say 

 nothing of their inter-relation, we as yet know far too little. 



