CHAP. II The Web of Life 29 



No illustration of the web of life can be better than the 

 most familiar one, in which Darwin traced the links of 

 influence between cats and clover. If the possible seeds in 

 the flowers of the purple clover are to become real seeds, 

 they must be fertilised by the golden dust or pollen from 

 some adjacent clover plants. But as this pollen is uncon- 

 sciously carried from flower to flower by the humble-bees, 

 the proposition must be granted that the more humble-bees, 

 the better next year's clover crop. The humble-bees, how- 

 ever, have their enemies in the field-mice, which lose no 

 opportunity of destroying the combs ; so that the fewer 

 field-mice, the more humble-bees, and the better next year's 

 clover crop. In the neighbourhood of villages, however, it 

 is well known that the cats make as effective war on the 

 field-mice as the latter do on the bees. So that next year's 

 crop of purple clover is influenced by the number of humble- 

 bees, which varies with the number of field-mice, that is to 

 say, with the abundance of cats ; or, to go a step farther, 

 with the number of lonely ladies in the village. It should 

 be noted, however, that according to Mr. James Sime there 

 were abundant fertile clover crops inNew Zealand before there 

 were any humble-bees in that island. Indeed, many think 

 that the necessity of cross-fertilisation has been exaggerated. 



Not all insects, however, are welcome visitors to plants ; 

 there are unbidden guests who do harm. To their visits, 

 however, there are often obstacles. Stiff hairs, impassably 

 slippery or viscid stems, moats in which the intruders 

 drown, and other structural peculiarities, whose origin may 

 have had no reference to insects, often justify themselves 

 by saving the plant. Even more interesting, however, is 

 the preservation of some acacias and other shrubs by a 

 bodyguard of ants, which, innocent themselves, ward off 

 the attacks of the deadly leaf-cutters. In some cases the 

 bodyguard has become almost hereditarily accustomed to 

 the plants, and the plants to them, for they are found in 

 constant companionship,' and the plants exhibit structures 

 which look almost as if they had been made as shelters 

 for the ants. On some of our European trees similar 

 little homes or domatia constantly occur, and shelter small 



