Page:Popular Science Monthly Volume 51.djvu/336

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THE POPULAR SCIENCE MONTHLY.

level of the field. Besides the moss or grass, they frequently employ coarse wax to form the ceiling of the vault, for the purpose of keeping out rain and preventing high winds from destroying it. Within this retreat the eggs present an appearance not very different from that of the bumblebee."

In conclusion, I may say that among the ancient Hebrews and Romans the error was widely credited that bees made their nests and reared their young in the carcasses of dead animals; and, although these people knew that bees were governed by a ruler, they labored under the impression that it was a king and not a queen. Such ignorance can easily be overlooked, however, when we come to consider that it is only of comparatively recent date that we have worked out the biology of these insects, and, as it is, there yet remains the greater part, by all odds, of their natural history of which we know little or absolutely nothing, and to which must still be added that of the host of species of this order yet to be discovered and made known to science.

 
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THE PRINCIPLE OF ECONOMY IN EVOLUTION.
By EDMUND NOBLE.

ONE of the many interesting things about evolution, oftener taken for granted than formally recognized, is the fact that the changes which everywhere accompany and constitute it have their rise in a simple excess of pressure in one direction over the pressure in another. For all movement, whether it be of simple or of complex matter, whether it be of an inorganic or an organic system, whether it involve will and conscious perception or not, is in every case and under every conceivable set of circumstances movement in a single mode — that is to say, movement in the direction of the least resistance, or from the direction of the greatest traction or stress.[1] If we look to the origin of the movement, we shall speak of acting as in the line of the greatest stress; if we consider the resistances in the presence of which movement is produced, we shall regard acting as in the direction of the least resistance. But, however we may describe it, the truth of the law is obvious, since it follows from the very nature of movement. For if a body be equally stressed from all directions it will not move, while if it be stressed differentially — in one direction more than in other directions — it will move in the line of, or away from, the greatest stress. Now, as all movement must

  1. In order to save repetition, the word "stress" will be used throughout in the sense of traction or stress."