4. The Baboon serves as an example of an animal walking on all four limbs in the “plantigrade” position, i.e., with the whole of the palms of the hands and the soles of the feet applied to the ground. 5. A small species of Antelope shows the characteristic form of a running animal, in which the limbs perform no office but that of supporting the body on the ground. This animal stands on the tips of the toes of its elongated, slender feet in the “digitigrade” fashion. 6. A Porpoise, adapted solely for swimming in the water. The fore-limbs are converted into flattened paddles, and the hind-limbs are entirely absent, their function being performed by the tail. The rudimentary pelvic bones are preserved. The rest of the case is occupied by details of the skull in some of its principal modifications. At the top are diagrams of the structure of bone and cartilage as shown by the microscope. [Illustration: FIG. 8.--THE SKELETON OF A FLYING-FOX, OR FOX-BAT (_Pteropus medius_). _cl_, clavicle; _cv_, cervical vertebræ; _d_, dorsal vertebræ; _fb_, fibula; _fm_, femur; _h_, humerus; _hx_, great toe, or hallux; _l_, lumbar vertebræ; _mc_, metacarpals; _mt_, metatarsals; _ph_, phalanges, or finger- and toe-bones; _pv_, pelvis; _px_, thumb, or pollex; _r_, radius; _s_, sacral vertebræ; _sc_, scapula; _sk_, skull; _tb_, tibia; _ts_, tarsus; _u_, ulna. ] In the wall-case on the opposite (north) side of the bay the study of the skeleton of Mammals is continued by illustrations of the structure of the limbs. At the top of the case is a diagram showing the correspondence of the hand and the foot in their complete typical form, with the names applied to the different bones. The series of specimens below shows the principal deviations actually occurring from this typical condition, which, as may be seen, is very nearly preserved in the human hand. One series shows some of the stages of modification for special purpose (specialisation) by which a typical five-fingered hand becomes converted into the single-toed fore-foot of the Horse; while another series ends with the fore-foot of the Ruminants, sometimes, but erroneously, called a “cloven hoof,” in which only two toes remain. Similar changes are shown in the toes of the hind-foot, illustrating the same common plan running through infinite modifications in detail, enabling the organ to perform such a variety of purposes, and to exhibit such diversity of outward appearance. The existence of this common plan is now generally regarded as due to inheritance from a common ancestor. [Sidenote: Teeth of Mammals.]