212 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



period of temporary parasitism, at the end of which there is a meta- 

 morphosis, they drop off from the fish into the mud, often far from 

 their birth-place. This is curious enough, but the idea of linkages 

 becomes incandescent in the mind when we note that, just as the fresh- 

 water mussel has young temporarily parasitic on fishes, so a fresh- 

 water fish, the bitterling (Rhodeus amarus), has its young temporarily 

 parasitic in the gills of the mussel. 



Life-histories of parasites. When we pass to parasites in a 

 stricter sense we find the most extraordinary interconnections, the 

 most widely separated animals often sharing a parasite between them. 

 Liver-rot, which has repeatedly killed a million sheep in a year in 

 Britain alone, 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. The tapeworm of the cat has its bladder-worm stage in the 

 mouse, the sturdie-worm of the sheep's brain has its tape-worm stage 

 in the dog, and similar relations hold for hundreds of species. The 

 troublesome threadworm of human blood (Filaria sanguinis hominis) 

 is transferred from man to man by the mosquito, and the guinea-worm 

 which was probably the fiery serpent that vexed the Israelites in the 

 desert, which passes into man in drinking-water, spends its youth 

 in a minute water-flea, called by the giant's name of Cyclops. The 

 importance of tse-tse flies in transmitting the minute animals which 

 cause sleeping-sickness and allied diseases is known to all. We have 

 spoken of the connection between cats and clover, and there is a not 

 less striking connection between cats and plague. For it seems to have 

 been shown in India that the more cats the fewer rats, and the fewer 

 rats the fewer rat-fleas, which are the agents in passing the plague- 

 germs to man. 



Far-reaching influence of certain animals; earthworms. We 

 realise the idea of the web of life in another way when we consider the 

 far-reaching influence of particular kinds of activity, the best instance 

 being the work of earthworms. In 1777 Gilbert White got at the very 

 root of the matter. " The most insignificant insects and reptiles are of 

 much more consequence and have more influence in the economy of 



nature than the incurious are aware of Earthworms, though in 



appearance a small and despicable link in the chain of nature, yet, if 



lost, would make a lamentable chasm Worms seem to be the 



great promoters of vegetation, which would proceed but lamely with- 

 out them, by boring, perforating, and loosening the soil, and rendering 

 it pervious to rains and the fibres of plants; by drawing straws and 



