THERE are many examples of the type of problem which is met with by the ecologist working in the field, some of which may be mentioned here. Epidemics of pneumonia occur regularly in China, killing many people. They are brought about by previous epidemics which have attacked wild marmots living in overcrowded conditions. The danger has been increased by the arrival of numbers of Russian immigrants and Chinese colonists who, ignorant of the danger from the marmots, have taken no precautions so that the infection has rapidly spread. In hotter countries, epidemics take the form of bubonic plague, which is transmitted by the bites

of fleas that have been infected with bacilli from rats and other rodents. The sugar-cane leaf-hopper was introduced by accident into the Hawaiian Islands from Australia, with the result that an immense amount of damage was caused to the sugar production. The damage was practically eliminated later by the introduction by Perkins of two Chalcid wasps from Queensland and Fiji, which parasitised the leaf-hopper, and by a Capsid bug which sucks its eggs.

By beating spruce branches in Windsor Forest, the writer recently discovered a sawfiy, of which only four examples had previously occurred in Britain. It is a well-known European species, and its caterpillar feeds on the spruce. It has been accidentally introduced into Canada and is doing an immense amount of damage to the spruce forests there. The Canadian government has granted a sum of money to employ an entomologist to study the sawfiy on the Continent, and to obtain any parasites which attack it. These are sent to Farnham Royal, where they are bred and thoroughly tested to see that nothing is sent which could become a pest in itself. They are then despatched to Canada to be used to eliminate the spruce sawfiy danger.

As an instance of how the intentional introduction of an animal into a strange country to compete with a pest there may be too hasty and the animal may become a pest in itself, mention may be made of the mongoose in Jamaica. It was originally introduced to kill the snakes, but rapidly increased, and when it had exterminated all the snakes, it took to killing all the birds, poultry, etc., and thus became as bad a pest as the snakes.