54 ANIMAL ECOLOGY 



sheep. No one in those days knew the causes or mechanisms 

 of transmission of either of these two diseases ; but at about 

 that time very large parts of the country were drained in order 

 to reclaim land for agricultural purposes, and this had the 

 effect of practically wiping out malaria and greatly reducing 

 liver-rot — quite unintentionally ! We know now that malaria 

 is caused by a protozoan which is spread to man by certain 

 blood-sucking mosquitoes whose larvae live in stagnant water, 

 and that the larva of the liver-fluke has to pass through one 

 stage of its life-history in a fresh-water snail (usually Limncea 

 truncatula). The existence of malaria depends on an abun- 

 dance of mosquitoes, while that of liver-rot is bound up with 

 the distribution and numbers of the snail. With the draining 



of land both these animals disappeared or became much 

 rarer.isb 



5. On the whole, however, we have been settled in this 

 country for such a long time that we seem to have struck a 

 fairly level balance with the animals around us ; and it is because 

 the mechanism of animal society runs comparatively smoothly 

 that it is hard to remember the number of important ways in 

 which wild animals affect man, as, for instance, in the case 

 of earthworms which carry on such a heavy industry in the 

 soil, or the whole delicately adjusted process of control of the 

 numbers of herbivorous insects. It is interesting therefore 

 to consider the sort of thing that happens when man invades 

 a new country and attempts to exploit its resources, disturbing 

 in the process the balance of nature. Some keen gardener, 

 intent upon making Hawaii even more beautiful than before, 

 introduced a plant called Lantana camaray which in its native 

 home of Mexico causes no trouble to anybody. Meanwhile, 

 some one else had also improved the amenities of the place 

 by introducing turtle-doves from China, which, unlike any of 

 the native birds, fed eagerly upon the berries of Lantana. The 

 combined effects of the vegetative powers of the plant and 

 the spreading of seeds by the turtle-doves were to make the 

 Lantana multiply exceedingly and become a serious pest on 

 the grazing country. Indian mynah birds were also intro- 

 duced, and they too fed upon Lantana berries. After a few 



