The next argument for Evolution to which we would refer is based on the similar structure and origin of organs or members that have entirely different uses. In Fig. 11 are figured the bones of the fore limbs of four different mammals, a whale, a bat, a dog, and man. The first is used for swimming, the second for flight, the third for locomotion on land, and the fourth as a grasping and holding organ. If these organs had been specially designed, each for its specific purpose, we should expect to find fundamental differences in structure. Actually the general arrangement of bones is the same in each case. A fact like this points strongly to a common origin of the four types mentioned, and to a general primitive arrangement of the bones of the limb. This primary type, it seems natural to suppose, has been modified for various special purposes in many different directions, the general features remaining recognisable. Many other cases of homology, or similarity of structure and origin, in organs whose function is dissimilar, might be quoted. Thus the poison gland of the poison snakes is not an organ which has been specially developed, but is a modified portion of one of the salivary glands. The hoof of the horse and the finger nail of man can evidently be satisfactorily explained as modifications of a general type of terminal claw, and the scales of the scaly ant-eater and the quills of the porcupine are only modified hairs. The significance of facts like these, when carefully considered, is very great. [Illustration: FIG. 11.--The bones of the fore limbs of (_a_) whale, (_b_) bat, (_c_) dog, and (_d_) man, showing essential similarity in arrangement.] [Illustration: FIG. 12.--Distribution of Marsupials or pouch-bearing animals. Australia, New Guinea, etc. 36 Genera. 144 Species. America 3 Genera. 28 Species.]