Page:Popular Science Monthly Volume 9.djvu/552

526 Most nocturnal beasts have large eyes, but most bats have very small ones.

This is perhaps due to the fact that bats in their flight are guided by an extraordinarily delicate sense of touch—so delicate as to seem almost like a sixth sense.

The external ear of most bats appears at first to be double—a very small one seeming to stand up inside the larger one. This



, however, is due merely to the very large development of a little piece which in ourselves projects backward as a small rounded process guarding externally the opening of our ear, and called the tragus.

The food of our English bats consists of insects, and their teeth bristle with sharp points, well suited to pierce the chitinous cases by which the bodies of insects are protected.

The stomach (like that of most beasts which live upon a purely animal diet) is a simple, short, and rounded bag.

The female is provided with a pair of milk-glands, situated on the breast—as in the apes and in man.

The skeleton of the bat, when compared with those of some other animals, affords an excellent example of how fundamental uniformity of structure may underlie forms which are strikingly different—in accordance with diverging habits of life.

I have already called attention to the divergent aspects of the aërial bat and the subterranean mole. Yet the bones of the flying-organ of the bat closely resemble those of the burrowing organ of the mole, save as regards the relative shapes and dimensions of the component bones. But, while in the bat these bones are drawn out into excessive length and tenuity, in the mole they exhibit the maximum of concentration and robustness. Now, both these conditions are but diverging manifestations of the human structure, and the same indeed may be said of such extreme modifications as the fore-leg of the horse or the paddle of the whale.

But the bat and the mole present us with a special point of similarity in their skeleton not found in the other animals named, including ourselves.

It is that the breastbone in both the bat and mole develops a median ridge or keel. This keel serves to afford additional surface for the attachment of powerful muscles which pass thence to the arms,