Page:Popular Science Monthly Volume 38.djvu/590

574 paraffin the material of the earth's crust and the same result will follow, but the limitations of the hole or heap will be different, because the strength of the materials is not the same. Assuming the earth to be homogeneous, the greatest possible stable prominence or depression is a measure of the strength of the material. Having examined a marked example of elevation and depression in the region of the Pleistocene Lake Bonneville and the WahsatchWasatch [sic] Mountains, the author deduces the working hypotheses that the measure of the strength of the crust is a prominence or cavity about six hundred cubic miles in volume; and that mountains, mountain ranges, and valleys of magnitude equivalent to mountains, exist generally in virtue of the rigidity of the earth's crust; continents, continental plateaus, and oceanic basins exist generally in virtue of isostatic equilibrium in a crust heterogeneous as to density.

The Dragon-fly and the Cricket.—Mr. E. Giles relates, in the Journal of the Bombay Natural History Society, that in June, 1888, his attention was attracted by a large dragon-fly which was cruising backward and forward in his porch in an earnest manner that seemed to show that he had some special object in view. Suddenly he alighted at a small hole in the gravel and began to dig vigorously, sending the dust in small showers behind him. "I watched him," says Mr. Giles, "with great attention, and after the lapse of about half a minute, when the dragon-fly was head and shoulders down the hole, a large and very fat cricket emerged like a bolted rabbit, and sprang several feet into the air. Then ensued a brisk contest of bounds and darts, the cricket springing from side to side and up and down, and the dragon-fly darting at him the moment he alighted. It was long odds on the dragon-fly, for the cricket was too fat to last, and his springs became slower and slower, till at last his enemy succeeded in pinning him by the neck. The dragonfly seemed to bite the cricket, who, after a struggle or two, turned over on his back and lay motionless, either dead or temporarily senseless. The dragon-fly then, without any hesitation, seized him by the hind legs, dragged him rapidly to the hole out of which he had dug him, entered himself and pulled the cricket in after him, and then, emerging, scratched some sand over the hole and flew away. Time for the whole transaction, say, three minutes."

Evolution in Floridian Shells.—Except Mr. Edward Potts's article on Fresh-water Sponges collected in Florida, all the papers in Vol. II (December, 1889) of the Transactions of the Wagner Free Institute of Science of Philadelphia are by Prof. Joseph Leidy. They are on Some Fossil Human Bones; Mammalian Remains from a Rock Crevice in Florida; Mammalian Remains from the Salt Mine of Petite Anse, Louisiana; Platygonus, an Extinct Genus allied to the Peccaries; and The Nature of Organic Species. The last paper relates to a series of shells found in Florida which appear to illustrate the transmission or evolution of an extinct form (Fulgur contrarius) into that of a living species (Fulgur perversus). The changes are illustrated by engraved plates. In this series, as also in a series of Strombus, great variability seems to have prevailed among the fossil forms, while the existing species are comparatively stable. Another shell, the Melongena coronata, found in the same bed, manifests great uniformity in structure; "while, at the present time, it is probably the most variable shell living on the coast of Florida. . . . We thus find in the same bed one genus that was widely variable in character which now manifests much greater stability in structure; and also two genera that were quite fixed or stable that at the present time are very inconstant." In explanation of this anomaly Prof. Leidy suggests that "no species has been found to be constant or permanent during a long period of geological time; and there appear to have been periods of rest and periods of activity in the transmutation of species. Surviving from the Miocene age, the Fulgur contrarius may have been ripe for a change, which was stimulated into action by a cause that would not affect other species, especially such as had not been in existence long. For the same reason the Melongena coronata and the Strombus pugilis may be active in their inconstancy now, as they have survived from a former period."