Page:Popular Science Monthly Volume 10.djvu/237

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upper surface of the fore-wings. It lies in the course and shape of the transverse posterior line—a line which is a general feature of ornamentation throughout this group of insects.

This position might be sustained by parallel facts, but the limit of this article does not allow of their being presented. It remains to see what light these observations throw upon the origin of all these different kinds of moths. They seem to me to point to a method of variation in this group, and to a reason for its display. And, if we can apply these observations to particular instances, they may lead to a better understanding of the value of these specific forms, by allowing us to appreciate with more exactness the amount of differentiation they have undergone in the lapse of time. In a wider sense, we may attempt a classification by the ornamentation and coloration in the moths based on method, and such studies cannot fail to lend fresh interest to the barren but necessary work of describing the different species or forms. It seems to me, also, that these observations vindicate the importance of studying the characters of color and pattern in the group, and are, perhaps, a criticism on the remarks of those writers who purposely allow these no higher value than the subordinate one of dividing their material into “species” in their collections. In fact, these characters may give us a clew to the genealogy of the group, and seem to be of sufficient importance to be noted in descriptions of genera.



THE conservation of energy, as treated in the most recent work of authority on the subject, Prof. Balfour Stewart’s, published in the “International Scientific Series,” regards energy as divisible into two classes, actual and potential: actual energy, as in the case of a rushing stream of water, or a radiant and contracting mass of molten iron; and potential energy, where, for instance, a stone is at a height and may fall, or where a spring is tightly coiled and may unwind.

Prof. Stewart speaks the general opinion of modern physicists, Profs. Rankine, Tyndall, and others, when he states that, in cases of potential energy, the bodies endowed with it are absolutely at rest, and that the motion exhausted in storing up such energy is represented by a mere advantage of position, which, when utilized, yields in actual, palpable motion the energy which has lain completely quiescent, as when the stone falls to the earth, or the freed spring uncoils.

This division of energy into actual and potential seems to me to