The Web of Life. 65 



ence of one creature upon another for the continuance of its kind. 

 Flowering plants and flower-visiting insects are bound up together 

 in intimate linkage. The insects visit the flowers for food ; in so 

 doing they secure cross-pollination, and the possible seeds become 

 real seeds. As Darwin pointed out — and his illustration is known 

 throughout the world — next year'.s crop of purple clover depends 

 in great part on the number of humble-bees in the district, which 

 varies with the number of voles or field-mice, which varies with 

 the abundance of cats fond of hunting. 



Not less important than the part which insects (and some 

 other creatures) play in the cross-fehilisation of flowers is the 

 function of birds and beasts as seed-scatterers, and here again 

 Darwin gives us the classic case of fourscore seeds germinating 

 from one clodlet on a bird's foot. The fertilised eggs of the 

 freshwater mussels develop within the outer gill-plate into minute 

 bivalve larvae, called Glochidia. These are liberated by the mother 

 when a freshwater fish, such as minnow or stickleback, comes into 

 the vicinity. The larvae fasten themselves to the fish, to which 

 they are constitutionally attracted. After a period of temporary 

 parasitism, the larvae undergo metamorphosis and drop off the fish 

 often far from their birthplace. The idea of linkages becomes 

 almost incandescent in our minds when we note that just as the 

 freshwater mussel has its young temporarily parasitic on fishes, so 

 a freshwater fish, the bitterling {Rhodeus amarus) has its young 

 temporarily parasitic in the gills of the freshwater mussel ! 



Some of the most extraordinary inter-linkages are to be 

 found in the life-history of parasites, — the most widely separated 

 animals often sharing a parasite between them. Liver-rbt in sheep 

 is due to a parasite which passes from sheep to water, from water to 

 water-snail, from water-snail to grass, from grass to sheep. Tsetse 

 flies disseminate the organisms of sleeping-sickness, mosquitos 

 those of malaria and yellow fever. The louse carries typhoid and 

 the dog-flea transmits bubonic plague. It seems to have been 

 shown in some parts of India that the more cats the less plague 

 (for the cats kill the rats), just as in Britain the more cats the more 

 clover. 



