Fig. 15 is introduced to show how easy may be the transitions from one order of animals to another. Two long, slender Lizards are shown and also a Snake. Bipes has four limbs, but they are very small and weak. Cheirotes has lost, probably through disuse, the hind-limbs and the front ones are small and weak, almost useless. The snake has no limbs at all. The two lizards are quite snake-like in external forms. Just as changing environment, use and disuse, etc., can effect transitions in external forms by slight gradations, so also may the whole internal structures undergo marked alterations by slight modifications through long ages. The brief references we have made to the changing life-forms during the geologic ages, should be very instructive even to a reader not familiar with zoölogy. They teach the important lesson that life, in the main, has ever advanced to higher and higher levels. At one time in the history of the earth no animals were in existence higher than the Invertebrates; these lived through the ages with ever-changing forms. Later, in the course of geologic history, some of the primitive Invertebrates evolved into the lowest Vertebrates, and Fishes finally appeared, at first very primitive and generalized. For long ages there were no creatures on the globe higher than these. Then as the æons slowly rolled on some of the early fishes evolved finally into a higher group of animals, the Amphibians, and these for long ages were the highest creatures in existence. As time slowly passed, some of the primitive amphibians evolved into primitive Reptiles, and these creatures for long ages were the highest in existence and monarchs of the world. Some of the primitive reptiles branched off in one direction leading to modern Birds, and some advanced in another direction that led through primitive mammals to man and other modern mammals. And now in modern times man is monarch among the animals. We may now begin to realize that truly with the changing and growing continents during the geologic ages, many life-forms have varied and advanced to higher levels, culminating in the final appearance of _man_ as well as other living creatures. SECTION V. NATURAL SELECTION. NATURAL SELECTION.