184 INSECT PESTS 



length. Nowadays nobody would willingly believe that 

 all or any of these repulsive creatures could be brought 

 into being by a long-suffering biped of the genus homo, 

 and indeed it has now been established that the air we 

 breathe, the water we drink, the dirt that gets on our 

 hands, and the food we eat, simply teem with microscopic 

 spores, ova and minute embryo forms which under favour- 

 able conditions might produce a serious state of things 

 for an unfortunate victim. Then the number of species 

 of parasites is hugely in excess of the number of hosts, so 

 that a terrible struggle for existence between the two 

 great classes is always going on. Incidentally there is a 

 borderland or neutral zone between them, vv^here parasite 

 and host live together on equal terms ; as, say, the hermit- 

 crab and the sea-anemone. This we call the " mess- 

 mate principle " where each is indispensable to the other. 

 Whether any such settlement of the pest question can 

 come about as a whole I am quite unable to say ; but if 

 so, I suppose caterpillars would have to be rationed, and 

 the marriage lines of butterflies officially approved by 

 the Ministry for Nature ! For the present we can only 

 take our share in the great struggle, which is what we are 

 here for. The reason why a negro has a thick skull and 

 an ebony skin is because his forebears fought and won a 

 battle against the fiercest sunhght that pours down on 

 this earth. The white man, pith helmet notwithstanding, 

 will never conquer Africa in that sense. He may, how- 

 ever, teach the negro not to live in squalor and dirt, 

 uplift his mind and civilize him, although there are wrong 

 as weU as right ways of doing that. The point is that 

 we humans, whatever our race and creed, are all liable 

 to the constant and unending change that is going on from 

 age to age all through creation in accordance \\dth the 

 change in the environment. This determines the char- 

 acter of both pest and host, and both alter from year to 

 year imperceptibly, but with a distinct tendency all the 

 time for one or the other to predominate. Take an ex- 



