In presenting Classification as the subject of a series of papers in the "Atlantic Monthly," I am aware that I am drawing largely upon the patience of its readers; since the technical nature of the topic renders many details necessary which cannot be otherwise than dry to any but professional naturalists. Yet believing, as I do, that classification, rightly understood, means simply the creative plan of God as expressed in organic forms, I feel the importance of attempting at least to present it in a popular guise, divested, as far as possible, of technicalities, while I would ask the indulgence of my readers for such scientific terms and details as cannot well be dispensed with, begging them to remember that a long and tedious road may bring us suddenly upon a glorious prospect, and that a clearer mental atmosphere and a new intellectual sensation may well reward us for a little weariness in the outset. Besides, the time has come when scientific truth must cease to be the property of the few, when it must be woven into the common life of the world; for we have reached the point where the results of science touch the very problem of existence, and all men listen for the solving of that mystery. When it will come, and how, none can say; but this much at least is certain, that all our researches are leading up to that question, and mankind will never rest till it is answered. If, then, the results of science are of such general interest for the human race, if they are gradually interpreting the purposes of the Deity in creation, and the relation of man to all the past, then it is well that all should share in its teachings, and that it should not be kept, like the learning of the Egyptians, for an exclusive priesthood who may expound the oracle according to their own theories, but should make a part of all our intellectual culture and of our common educational systems. With this view, I will endeavor to simplify as far as may be my illustrations of the different groups of the Animal Kingdom, beginning with a more careful analysis of those structural features on which classes are founded.