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AGAZINES are of two kinds: the entertaining kind, which cater to mental laziness, and can be skimmed over without mental effort; and the instructive kind, which contain articles that must be read twice. "The Popular Science Monthly," from the start, has furnished a considerable proportion of articles so weighted with valuable thought as to require concentration of mind and often careful re-perusal to grasp and assimilate their contents. This has provoked frequent protests on the part of our readers, who have complained that we deviate from the magazine-standard of easy reading, and are not true to our title, which promises a magazine adapted to the populace. Yes, but to the improvement of the populace! The "Monthly" was started for no other reason than because the standard of our current magazines was low—too low to reflect the best mental activity of the times. Their aim is to amuse and beguile their readers by calling forth the smallest possible amount of mental reaction. We started because the popular magazines, competing downward, shirked vigorous work, and were false to the demands of an age characterized beyond all others for its intellectual seriousness, and by the magnitude, importance, and practical quality of the questions that are occupying the ablest minds in all countries. These minds can not be followed—these questions can not be understood without effort on the reader's part. This is the price that must be paid for real knowledge. People can not be amused into mental grasp and vigor, they must be exercised into it. We talk of mental progress, mental elevation, mental expansion, but these are attainable only on the condition of mental exertion.

We print, in two parts, another of those articles that have to be re-read to get their full import. It is by Professor Du Bois-Reymond, and is on "Exercise," so that it is at the same time an illustration and an exposition of our subject. It is a most original and instructive statement, and will well repay re-perusal.

One of the inevitable effects of the advancement of science in various directions is the establishment of new connections of thought which are often most striking and significant. What can Darwinism have to do with exercise? Restricting the term Darwinism, as we must do, to natural selection, Du Bois-Reymond shows that they are very closely related. Viewing organic nature mechanically, the series of living beings has been unfolded during unlimited time by adaptation to new conditions, the course of movement being in an ascending scale. "From this point of view, organic nature appears not only as a machine, but also as a self-improving machine." But the law of self-improvement is, that powers and faculties are strengthened and grow by exercise, and are weakened by non-exercise. In the struggle for existence, therefore, those will win and survive in whom exercise has developed superior capacities and resources, while the less exercised and weaker fail and perish. The principle of exercise is thus a kind of motive-power in animal evolution, and, as might be expected, is full of the most important results in the higher spheres of physiological and psychical activity.