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426 if it cracks, that it has been bleached too much. The thickness of paper can be measured by putting a number of leaves together, or by the micrometrical determination of the effect of adding a single leaf to the mass. We may burn the paper and examine the ashes. If they amount to more than three per cent, clay, kaolin, spar, or gypsum has been added to the pulp. When we color paper with an iodine solution, yellow indicates the presence of wood-fiber; brown, of cotton or linen; and the absence of coloration, of cellulose.

Man's Agency in the Extermination of Species.—Extermination is defined in Nature as indicating that in certain parts of the range of a species, whether plant or animal, it has ceased to exist, however abundant it may remain elsewhere; while in other cases, especially if the species have but a limited distribution, it easily becomes equivalent to extirpation. The older school of zoölogists seem hardly to have contemplated the possibility of a whole species having become extinct within the period since man appeared upon earth, or to have supposed that a species could by human efforts be utterly swept away. Thus there was once skepticism about the extinction of the dodo, or, that having been established, about its having existed within the human period. The disappearance of numerous animals, formerly abundant, from the settled parts of our country, affords examples of local extinction; and the fate of the buffalo threatens to furnish an instance of total extinction by the agency of man. Man's agency usually acts indirectly—as by changing the conditions of the country, so as to make them unfavorable to the subsistence of certain animals, rather than directly by killing all the individuals of a species outright. The wolf has defied all efforts, by offering bounties and otherwise, to accomplish its destruction in Europe, except in artificially-built-up Holland, where it never was at home; Denmark, every spot of which is accessible to the hunter; and the United Kingdom, where its forest resorts have been removed. Other instances are the extirpation of the quail in New Zealand by means of fires that were lighted for other purposes; the threatened destruction of other interesting animals of Australia and New Zealand by animals of the weasel kind that were introduced to prey upon the imported rabbits; and the destruction of turkey-buzzards' eggs and petrels in Jamaica by the mongooses that were taken there to make war upon rats; of the Diablotin petrel of Dominica by a species of opossum; and the destruction of the cahows in the Bermudas, till it is not known now whether the bird exists there. The great skua, or "bonxie," disappeared from one of its three breeding-stations in the Shetland Islands several years ago, and has been maintained at the other two only through the vigorous exertions, to repress poachers and preserve it, of the late Dr. Robert Scott and the late Dr. Lawrence Edmondston, respectively. The Zoölogical Society has ordered medals struck in honor of the services these gentlemen rendered to science. Though the reward is posthumous, and goes to the heirs of the well-doers instead of to themselves, the acknowledgment is a fitting one, marks an example, and is an encouragement to the lovers of living nature.

Prof. Wright in the British Association.—Prof. G. F. Wright's paper in the British Association, on The Ice Age of North America and its Connection with the Appearance of Man on that Continent, is spoken of in Nature as a most interesting one. The author said that the glacial deposits, transported from several centers mostly outside the Arctic Circle, and the absence of a polar ice-cap, militated against an astronomical and for a geographical cause of the great cold, particularly as an uplift of the glaciated area was coincident with an important subsidence in Central America. He regarded the so-called "terminal moraine of the second period" as a moraine of retreat due to the first glaciation, and thought that the evidence of forest beds, mainly to the south of the area, indicated local recessions of ice, and not a single great interglacial epoch. Palæolithic remains similar to those of the Somme and Thames have been found in several gravel terraces flanking streams which drain from the glaciated region, and made up of glacier-borne detritus; they are regarded by the author as deposits of the floods which characterized the closing portions of the Glacial period. The recession of the falls of Niagara and St. Anthony