Page:Popular Science Monthly Volume 45.djvu/111

Rh the control of that community of impulses and purposes, and that consensus of moral ideas and perceptions, which we call public conscience. This influence is beginning to penetrate even the darkest regions of Central Africa and to protect the unknown barbaric tribes against the ravages of Arab slave traders and the arbitrary authority of European adventurers. Each nation that joins in this combined movement is doubtless seeking, first of all, to further its own commercial and colonial interests; but it suffices as an illustration of the prevailing spirit of the age that the basis on which they profess to unite is the broad principle of a common humanity.



T is probable that the sleep or dormant period which mollusks share in common with many other organic beings is brought on not merely by the exigencies of climate, but that it is more or less necessary in building up the wasting physical powers. All organized beings seem to require rest in some form or other. If plants, whether from the tropics or temperate regions, are kept in hothouses, they will not grow the year round, and when forced to do so soon become sickly or die outright.

With the mollusks this sleep in many cases may be prolonged indefinitely, often without the slightest apparent damage, and under some conditions which seem really astonishing.

In the sea the clams (Venus and Mya) have rest periods, during which they sink more deeply into the mud and retreat from the fisherman; the tritons, murices, and ranellas form a shelly growth and mark their seasons of repose by a thickening of the aperture called a varix, which is sometimes guarded by spines or knobs. The littorinas, which are amphibious, pass most of their time on grass or sedges at the edge of the sea in the colder regions and high aloft on mangrove or other trees in the tropics, only occasionally going into the water to moisten themselves. Tryon tells of some West Indian species which survived over a year in his cabinet, and of others that lived for months in the dry air of Philadelphia, though they exhibited but little activity; and the writer has kept specimens of the nearly allied Tectarius alive in his collection for nearly two years.

Most of the fresh-water species of mollusks pass a period of hibernation in cold climates or testivation in the tropics, and many of them are wonderfully tenacious of life when withdrawn from the water. In June, 1850, a living pond mussel was sent to Dr. Gray from Australia which had been kept out of water more 