Our limbs form beautiful subjects for comparison. Throughout the vertebrates they never exceed four in number. They are all modifications of one type, whether we take the fins of fish, the wings and legs of birds, fore and hind legs of quadrupeds, or arms and legs of man. Comparing the leg of a bird with the leg of a man, we see that the complete leg of a bird shows first the thigh bone, then the tibia or lower leg bone, and then in the place of the tarsus and metatarsus a single bone, with, at its lower extremity, a small bone supporting the four toes. Primarily the analogy between the last five bones of the bird and the so-called tarsus, metatarsus, and toes of man does not seem very complete, but if the chick in the egg, be examined, its leg will be found to consist of the thigh bone, of the tibia, of two tarsal and three or four metatarsal bones, and the toes or phalanges. The upper tarsal bone subsequently becomes anchylosed with the tibia and the lower one with the consolidated metatarsus. Now the analogy becomes much more complete.
The horse has but a single metatarsal bone (the third), with rudiments of the second and fourth. These rudimentary metatarsal bones of the horse are very interesting. By means of them it is comparatively easy to trace out his descent. I may be pardoned for mentioning such well-known facts and analogies as the following, among the vertebrata—that the whale possesses the rudiments of hind legs, that the boa constrictor possesses also the rudiments of a leg and a pelvis, and that the rudiments of the wings are discoverable in the apteryx.
A few other animal analogies: The third eyelid of the bird exists also in some amphibians and reptiles and in sharks; also in man as a rudimentary structure.
The manner in which cows, deer, and sheep tear up the grass when they are feeding, plucking away at the tufts, is familiar to any observant man. The incisors of the upper teeth are wanting. The interesting analogy is the fact that the teeth are really there, but they are uncut—that is to say, they have never pierced the gum.
The skin with its appendages forms a beautiful story of analogy. Our own microscopical epidermic scales are. strictly comparable with the cells that make up the scales of fish and of reptiles; their further development into hairs and, nails again compares with the feathers of birds and the hooves and horns of animals.
We call ourselves a hairless race; but everywhere on our bodies are the small lanugo hairs. Stimulation will readily cause these hairs to grow to any extent The surgeon has frequent opportunities of witnessing this retrograde progression toward a lower type.
Molting has its analogy throughout the animal kingdom. We
