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. i 
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 cf 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 
