Page:Popular Science Monthly Volume 32.djvu/442

442 represent rivers or inlets of the Tertiary sea; that they are the beds of extinct lakes; and that they represent the beds of Quaternary rivers. Mr. R. T. Hill, who has made a geological examination of the region, finds all these theories wrong. He sees in the ground on which the timbers grow, the detritus of arenaceous strata which occupy well-defined horizons in the geologic series, and which have been exposed by the denudation of the overlying strata. The timber confines itself to these arenaceous belts because they afford a suitable matrix for the penetration of the roots of trees and a constant reservoir for moisture; while "the barrenness of the prairies, so far as forest growth is concerned, is owing to the absence of the requisite structural conditions for preservation of moisture as well as to the excess of carbonate of lime in their soils."

Injuries caused by Parasitic Fungi.—The injuries which parasitic fungi produce upon their host plants, were described in the American Association by A. B. Seymour. Parasites take away the nutriment of the plants, killing or continually absorbing the food-supply of individual cells, and injuring cell-walls. They impair the power of assimilation, weaken the physiological power, causing the formation of spots and of black molds to obstruct the passage of light. Some fungi cause a change of position in their host, and less favorable exposure. They provoke abnormal acceleration or retardation of growth, with resultant distortion and impaired vitality. Any part of the plant may be affected. In many grasses the entire inflorescence is destroyed. Decay is produced in ripe fruits, and valuable plants become infected with disease from less valuable ones. The extent of the injury attributable to any one cause is hard to determine, because several causes act together.

Deceptive Sensations.—The evidence of our senses is correct in nearly all cases in which two of them are called into play, so that the testimony of one is checked by that of the other. But when we have to rely upon one sense alone, we are sometimes liable to curious deceptions. This is the case with some of the feelings of touch. In hip-joint disease the pain is often referred to the knee, while it is really in the hip. This is because the nerve which conveys sensation from the knee, also sends a branch to the hip-joint. The experiences of those who have lost a limb are familiar. For some time afterward they feel sensations and pains of all sorts in the member that is gone, 60 that they can hardly convince themselves that it is not still there, itching or aching or smarting. This is because the nerve which used to convey feeling to the lost extremity is affected by some temporary accident. The feeling, which may be real, as to the trunk that remains, is still, by force of habit, referred to the extremity whence it used to come. Another experiment in deceptive sensations may be made by crossing the second finger over the first, and then placing a marble between the tips of the fingers, when it will be almost impossible to convince one's self that there are not two marbles. This is because two points in the fingers are touched simultaneously, which in the ordinary position could only be touched at the same time by two marbles. Acting upon its previous knowledge, the brain says that there are two.

Relative Mortality of Social Classes.—Mr. Noel A. Humphreys, in a paper on the relative mortality among the different classes in society, after citing the general evidence of the British life-tables that the mean duration of life has perceptibly and steadily increased in recent years, shows that this factor is mainly controlled by the rate of mortality in childhood; and the expectation of life is regarded in the life-tables as greater at ten years of age than it is at birth. Subsequently to childhood, the greater vitality of the upper and middle classes, compared with that of the general population, is only somewhat less marked than it is at under five years of age. The statistics of mortality, according to occupation, show a great difference in favor of quiet pursuits, and, among workingmen, of those engaged in the country as against those occupied in cities. Many of these differences are doubtless due more to the influences and risks of occupation than to the mere influence of