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THE POPULAR SCIENCE MONTHLY. 



paraffin the material of the earth's crust 

 and the same result will follow, but the 

 limitations of the hole or heap will be dif- 

 ferent, because the strength of the mate- 

 rials is not the same. Assuming the earth 

 to be homogeneous, the greatest possible 

 stable prominence or depression is a meas- 

 ure of the strength of the material. Hav- 

 ing examined a marked example of elevation 

 and depression in the region of the Pleisto- 

 cene Lake Bonneville and the Wahsatch 

 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 pla- 

 teaus, 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 Bom- 

 bay 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 man- 

 ner that seemed to show that he had some 

 special object in view. Suddenly he alight- 

 ed 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 dragon- 

 fly seemed to bite the cricket, who, after a 

 struggle or two, turned over on his back 

 and lay motionless, either dead or tempo- 

 rarily senseless. The dragon-fly then, with- 

 out any hesitation, seized him by the hind 

 legs, dragged him rapidly to the hole out of 



which he bad 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." 



Evolntion 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 Trans- 

 actions 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, Louisi- 

 ana ; Platygonus, an Extinct Genus allied 

 to the Peccaries ; and The Nature of Organ- 

 ic Species. The last paper relates to a se- 

 ries 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 Mchngena 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 in- 

 constant." 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 transmu- 

 tation of species. Surviving from the Mio- 

 cene age, the Fulgur -contrarius may have 

 been ripe for a change, which was stimu- 

 lated 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 Melongcna coronata and the 

 Strombus pugilis may be active in their in- 

 constancy now, as they have survived from 

 a former period." 



