The ears of the frogs and all higher animals are, like the tongue-bone and the lower jaw, derived originally from portions of gills, which the aquatic ancestors of living animals used to draw the oxygen from the water. This is one of the most wonderful and interesting changes which the study of evolution has unfolded to our knowledge. The disproportionate voices are produced by means of an extra amount of skin on the throat, which is distensible and acts as a drum to increase the volume of sound. In certain bullfrogs which grow to be as large as the head of a man, the bellowing power is deafening and is audible for miles. In Chile a small species of frog, measuring only about an inch in length, has two internal vocal sacs which are put to a unique use. Where these frogs live, water is very scarce and the polliwogs have no chance to live and develop in pools, as is ordinarily the case. So when the eggs are laid, they are immediately taken by the male frog and placed in these capacious sacs, which serve as nurseries for them all through their hatching and growing period of life. Although there is no water in these chambers, yet their gills grow out and are reabsorbed, just as is the case in ordinary tadpoles. When their legs are fully developed, they clamber up to their father's broad mouth and get their first glimpse of the great world from his lower lip. When fifteen partly developed polliwogs are found in the pouches of one little frog, he looks as if he had gorged himself to bursting with tadpoles. To such curious uses may vocal organs be put. Turtles are voiceless, except at the period of laying eggs, when they acquire a voice, which even in the largest is very tiny and piping, like some very small insect rather than a two-hundred-pound tortoise. Some of the lizards utter shrill, insect-like squeaks. A species of gecko, a small, brilliantly coloured lizard, has the back of its tail armed with plates. These it has a habit of rubbing together, and by this means it produces a shrill, chirruping sound, which actually attracts crickets and grasshoppers toward the noise, so that they fall easy prey to this reptilian trapper. So in colour, sound, motion, and many other ways, animals act and react upon each other, a useful and necessary habit being perverted by an enemy, so that the death of the creature results. Yet it would never be claimed that the lizard thought out this mimicking. It probably found that certain actions resulted in the approach of good dinners, and in its offspring this action might be partly instinctive, and each generation would perpetuate it. If it had been an intentional act, other nearly related species of lizards would imitate it, as soon as they perceived the success which attended it.