The rattles from which these serpents take their name, are a number of hollow, horny, button-like structures at the tip of the tail, which rattle together, with a peculiar humming sound, when the creature shakes its tail, as it is sure to do when disturbed or angry. It thus gives a warning to the man who might not have noticed the sluggish creature in his path in time to jump aside. Not all of the tribe have a rattle, however; and one of the reasons why our water-moccasin and copperhead are so much dreaded is that they possess no rattle, and therefore sound no "keep-off" warning. All our American venomous snakes are too heavy and slow to climb trees. They get their prey--mice, gophers, snakes, etc.--by going to a place where it is likely to be running about, and then patiently waiting until something comes within striking distance. CHAPTER XXIX AMPHIBIANS You will remember that the amphibians are distinguished from the true reptiles by having to pass through a tadpole stage before they obtain their perfect form. A good example is the frog, which in one kind or another exists in all parts of the earth except the very coldest. No doubt, you have often seen great masses of its jelly-like spawn floating on the surface of ponds early in the spring; and you must have wondered how such small creatures as frogs could possibly lay such enormous batches of eggs. But the fact is that when these eggs are first laid they are very tiny. Each egg is only about as big as a small pin's head. Instead of having shells, however, they are covered with a very elastic skin, while at the same time they soak up water. So, as soon as they pass into the pond they begin to swell, and very soon each egg is as big as a good-sized pea. TADPOLE AND FROG In the middle of each egg is a round black spot, which increases in size every day. This is the future tadpole, and after a time the egg-skin splits, and out it tumbles into the water. It is an odd-looking creature--just a big round head with a tiny pair of gills and a little wavy tail, and nothing else at all. But it manages to swim by wagging its tail, and it feeds on the tiny scraps of decaying matter which are always floating about in the water of the pond. Before long a little pair of legs begin to show themselves just at the base of the tail. A few days later another pair begin to grow in front of them. Then, by slow degrees, the tail passes back into the substance of the body, and so do the gills, while lungs are developed and nostrils are opened. And by the time that all these changes have taken place the tadpole has ceased to be a tadpole and has turned into a frog.