MOSQUITOES AND FLIES
there are thousands of species of flies that do not affect us in any injurious way; while, furthermore, there are species, and many of them, that render us a positive service by the fact that their larvae live as parasites in the bodies of other injurious insects and bring about the destruction of large numbers of the latter.
Scientifically, the Diptera are most interesting insects, because they illustrate more abundantly than do the members of any other order the steps by which nature has achieved evolution in animal forms. An entomologist would say that the Diptera are highly specialized insects; and as evidence of this statement he would point out that the files have developed the mechanical possibilities of the common insect mechanism to the highest general level of efficiency attained by any insect and that they have carried out many lines of special modification, giving a great variety of new uses for structures originally limited to one mode of action. But when we say that any animal has developed to this or that point of perfection, we do not mean just what we say, for the creature itself has been the passive subject of influences working upon it or within it. A fundamental study of biology in the future will consist of an attempt to discover the forces that bring about evolution in living things.
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