“It would be impossible for me here and now to give you the details of this method of investigation, but I can tell you enough to illustrate my statement. Take a homely and very familiar example, that of the branch of Articulates. Naturalists divide this branch into three classes,—Insects, Crustacea, and Worms; and most of them tell you that Worms are lowest, Crustacea next in rank, and that Insects stand highest, while others have placed the Crustacea at the head of the group. We may well ask why. Why does an insect stand above a crustacean, or, _vice versa_, why is a grasshopper or a butterfly structurally superior to a lobster or a shrimp? And indeed there must be a difference in opinion as to the respective standing of these groups so long as their classification is allowed to remain a purely arbitrary one, based only upon interpretation of anatomical details. One man thinks the structural features of Insects superior, and places them highest; another thinks the structural features of the Crustacea highest, and places them at the head. In either case it is only a question of individual appreciation of the facts. But when we study the gradual development of the insect, and find that in its earliest stages it is worm-like, in its second, or chrysalis stage, it is crustacean-like, and only in its final completion it assumes the character of a perfect insect, we have a simple natural scale by which to estimate the comparative rank of these animals. Since we cannot suppose that there is a retrograde movement in the development of any animal, we must believe that the insect stands highest, and our classification in this instance is dictated by Nature herself. This is one of the most striking examples, but there are others quite as much so, though not as familiar. The frog, for instance, in its successive stages of development, illustrates the comparative standing of the orders composing the class to which it belongs. These orders are differently classified by various naturalists, according to their individual estimate of their structural features. But the growth of the frog, like that of the insects, gives us the true grade of the type.[10] There are not many groups in which this comparison has been carried out so fully as in the insects and frogs; but wherever it has been tried it is found to be a perfectly sure test. Occasional glimpses of these facts, seen disconnectedly, have done much to confirm the development theory, so greatly in vogue at present, though under a somewhat new form. Those who sustain these views have seen that there was a gradation between animals, and have inferred that it was a material connection. But when we follow it in the growth of the animals themselves, and find that, close as it is, no animal ever misses its true development, or grows to be anything but what it was meant to be, we are forced to admit that the gradation which unquestionably unites all animals is an intellectual, not a material one. It exists in