The communists of western Europe, who would like to do the same thing without massacres and foreign invasions, are accused of trying to produce the chaos which they want to prevent. Probably the best demonstration of natural selection at work is that of Dubinin and his colleagues. He showed that within a species of fly, a particular type increased during warm weather, and decreased in the winter. By laboratory experiments he showed that the type which increases during the warm weather is much more easily killed by cold than the other type. However, a really thorough study will require counts of the numbers of an animal species over many years, including a study of the reasons why members of it die at different periods. The gall midges have to rely on birds to eat their parasites, even if a great many of themselves are eaten along with them.