9 8 



THE YOUNG NATURALIST. 



[May, 



to explain the insect's appearance here in Cheshire, where the larvae 

 seem to have been in greater profusion than -on the south coasts 

 (where by this theory one would expect them to have been in much 

 larger numbers), we must assume that impelled by some unerring 

 instinct, in comparison with which that of the swallow or " the prud- 

 ent crane " were mere foolish blundering, certain individuals who, we 

 must remember, could by no possibility, from the very nature of the 

 case, have had the slightest previous experience either inherited or 

 acquired, made direct right across England for certain spots in Ches- 

 hire and elsewhere where their particular food-plant was in abundance. 

 I think Mr. Corbett's quotation from Euclid would apply to such an 

 assumption. 



If on the contrary, we give up the theory of voluntary migration 

 and fall back on wind-blown immigration, we are no better off, for 

 then the enormous numbers of the host which must originally have 

 left some other land in order that the few spots which by the presence 

 of their food-plants could have intercepted and arrested them in such 

 large numbers as we see, appears quite incredible and could not pos- 

 sibly have escaped observation. I think the same reasons slightly 

 modified will apply to Edusa, although there is no doubt that the 

 agency of southerly and easterly winds does account for certain 

 isolated instances of rare insects found sporadically every season 

 along the south coasts. 



If then neither retardation of development nor immigration seems 

 an adequate explanation of these strange phenomena, what other 

 theory can be adduced. I think the whole subject is too little under- 

 stood to dogmatize about it, and it strikes me that if we knew more of 

 the various contingencies which influence and assail the embryonic 

 life of all insects, we should see our way more clearly to an explana- 

 tion. Entomologists know that almost every species of insect we 

 have is subject to great fluctuation in appearance. We have Antiopa 

 years, years when Coccinella do more especially abound, and seasons 

 when every other insect seems to be an earwig ; and it seems to me 

 that the secret of all these variations lies in the mutual correlation 

 of two factors: (i.) That all insects are individually so prolific that 

 if the majority of ova in each year of any species came to perfection, 

 each year would witness such abundance as now strikes us by its 

 variety. (2.) That the destruction of immature insect life is so 

 enormous that in normal years only the smallest fraction of original 

 ova ever see the imago state. 



Moreover the forces arrayed against this immature life are as 



