Page:Popular Science Monthly Volume 15.djvu/33

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23
CLEWS IN NATURAL HISTORY.

whales of to-day — a creature this, of which, as regards its teeth at least, modern whales are but shadowy reproductions. While under the shelter of great authority we may declare this ancestor of the whale to have been intermediate in nature between the seals and whales, or between the whales and their neighbors the manatees or sea-cows and dugongs. In either case, the intermediate character of the animal argues in favor of its having been the likely parent of a race dentally degraded in these latter days.

There is little need to specialize further instances of the occurrence of rudimentary organs in the higher animals, save to remark that not the least interesting feature of such cases is contained in the fact that the milk-glands of male animals among quadrupeds — organs which exist in a rudimentary condition — have been known to become functionally active and to secrete milk; this peculiarity having been known to occur even in the human subject. Among the higher quadrupeds, however, there yet remains for extended notice one special instance of the occurrence of "rudimentary organs," wherein, not merely is the nature of the parts thoroughly determined, but the stages of their degradation can be clearly traced through the remarkable and fortunate discovery of the "missing links." Moreover, the case in point, that of the horse, so clearly illustrates what is meant by progressive development or evolution of a species of animals, that it is highly instructive, even if regarded from the latter point of view.

frameless
Fig. 8

When we look at the skeleton of a horse's fore-limb, we are able, without much or any previous acquaintance with the facts of comparative anatomy, to see that it is modeled upon a type similar to that of the arm of man. Were we further to compare the wing of the bird, the paddle of the whale, the fore-limb of the bat, and the fore-leg of a lizard, with the equine limb, we should find the same fundamental type of structure to be represented in all. Thus we find in the arm of man (Fig. 8) — to select the most familiar example from the series just mentioned — a single bone, the humerus (3), forming the upper arm; two bones {radius (4) and ulna (5) constituting the fore-arm; eight small bones forming the wrist (carpus): five bones — one for each finger — forming the palm or metacarpus and five fingers, each composed of three small bones, named phalanges, with the exception of the thumb, in which, by a mere inspection of that digit, we may satisfy ourselves only two joints exist. In the wing of the bird (Fig. 9) we find similarly an upper-arm bone or humerus, two bones (radius and ulna) in the forearm; a wrist (b), a thumb (g), and two fingers (c f e d). Now, turning to the fore-limb of a horse (Fig. 10) — the hind-limb being essentially similar, in its general conformation, and corresponding as closely with man's lower limb — we find its conformation to correspond in a remarkable fashion to that