2IO LIFE : OUTLINES OF GENERAL BIOLOGY 



Iwttoms arc a naked sand". He quaintly adds: "Thus Nature, who 

 is a great economist, converts the recreation of one animal to the 

 supjwrt of another." How interested Gilbert White would have 

 been in the modern disclosure of the part that Japanese cattle, 

 standing in the water, i)lay in the spread of the formidable human 

 parasite called Bilharzia — but that is certainly another story. 



Nothing lives or dies to itself. The naturalist of Sellx)rnc noted 

 that trcK)ps of water-wagtails run al)out the cows that are feeding 

 in moist low pastures, and pick up the flies attracted to the cattle, 

 or the other insects disturlx'd by their hoofs. How interested he 

 would have lx*en in the now familiar fact that these water-wagtails 

 check the spread of liver-rot in sheep, since they are particularly 

 fond of the little water-snail {Limn cms truncaiulus), which is the 

 host of the larval stages of the liver-lluke that causes the sometimes 

 serious "rot". 



Further illustrating this central ecological idea of inter-rela- 

 tions, let us notice some of the results that follow the reduction 

 of wild corners throughout the country. From the farmer's point of 

 view there is. no doubt, much to be said for trimming hedgerows 

 and doing away with weedy borders and wild corners in fields, for 

 thes<.' are often the haunts and nurseries of insects injurious to crops. 

 Nevertheless, the other side of the account must not be overlooked, 

 that hedgerows and the like afford shelter or cover to birds and 

 other creatures, many of which are conspicuously or inconspicuously 

 the farmer's friends. The more garden-like a countryside becomes, 

 the greater is the risk that useful comjwncnts of the fauna will 

 (lisiipjx'ar for lack of shelter. Tidiness may easily be carried too far! 



THE CHANGES GOING ON.—Both Gilbert White and Charles 

 Darwin realised, as we have said, the cumulative importance of 

 little things; and this must be kept in mind in connection with 

 changes in the countryside going on to-day. A particular reduction 

 of shelter or cover may seem quite trivial, but the sum-total of 

 results, over a large area and for a long stretch of years, may be 

 of far-rtarhing ini|x)rtance. Another preliminary caution is that 

 there are few changes which are one-sided in their results; most 

 arc jKirtly against and partly for human interests. We have no 

 warrant for e.xjxcting that Nature — "friendly" as she often is — 

 should always ojKrate in man's favour. Thus, while there are a few 

 birds, like wood-pigeons, that are almost wholly on the minus side 

 as far as agriculture is concerned, and while the great majority are 

 almost altogether to the good, of not a few it must be said that if 

 they do some harm, they also do much good. There are famihar 

 and long-standing arguments, pro and con, in regard to such birds 

 as rooks, such mammals as moles, such insects as wasps. In many 

 cases, like the last-mentioned, the data are not yet abundant and 



