For some years it has been recognized that the infection comes from the bite of a single species of sand fly—a vicious pest smaller than a mosquito. Protection is afforded only by special screens. Ordinary mosquito netting is worthless. The death belt is a place of bright sunshine nearly every day. The insects cannot endure light. They remain secluded and it is difficult to secure specimens, even when the hiding places are known. As soon as darkness comes they emerge in enormous numbers. Harvard entomologists who investigated the death belt a few years ago spent the hours between sunset and sunrise in a specially screened railroad car. A few moments outside might have proved fatal. Due to some delicate balance of nature this sand fly seems to be confined almost exclusively to this locality. It is credited with causing about 7,000 deaths in the decade before the last war. _Enigma of Evolution: the Snake_ Snakes once had legs. There is evidence in their anatomy that they are descended from four-legged land animals. This evidence is found especially in certain bones near the base of the tail of one of the largest of living snakes, the python, which is the most primitive of the order and presumably nearest to the hypothetical ancestor. Although the snake remains an enigma of evolution, there is no doubt that it got rid of its legs because they were a distinct hindrance to its peculiar ways of life. The serpent is not very ancient, as animal types go. Evidently it first appeared in the Cretaceous geological period, about 100,000,000 years ago, when the great dinosaurs were the earth’s dominant animals. There are, however, no unquestioned fossils of snakes from the dinosaur days. The first snake-like creature known is represented by fossils from the Eocene, or “dawn”, age in North America. This was quite lizard-like in bone structure. It lived about sixty million years ago, when mammals were developing on earth. Rocks in Germany, laid down about twenty million years later, yield fossils of true snakes of the generalized viper type. Sometime later come fossils of snake giants from Egypt. Some of these probably were sixty feet long. But all these were real snakes, with no traces of external limbs. The ancestor seems lost forever because snake skeletons are brittle and delicate and do not easily fossilize. Having discarded legs, serpents evolved means of locomotion suitable to their ways of life. This has sometimes been described as “walking on the ribs.” It requires a highly intricate coordination of ribs and muscles and can be compared best to rowing a boat.