A very slight incursion into biological fields may serve to show forth the difficulties of naturalists when the task of separating animals from plants is mooted for discussion. To begin with, if we suppose our popular disbeliever to assert that animals and plants are always to be distinguished by shape and form, it is easy enough to show him that here, as elsewhere, appearances are deceptive. What are we to say of a sponge, or a sea anemone, of corals, of zoophytes growing rooted from oyster shells, of sea squirts, and of sea mats? These, each and all of them, are true animals, but they are so plant-like that, as a matter of fact, they are often mistaken by seaside visitors for plants. This last remark holds especially true of the zoophytes and the sea mats. Then, on the other hand, we can point to hundreds of lower plants, from the yeast plant onward, which show none of the ordinary features of plant life at all. They possess neither roots, stems, branches, leaves, nor flowers, so that on this first count of the indictment the naturalist gains the day. Power of movement, to which the popular doubter is certain to appeal, is an equally baseless ground of separation. For all the animals I have above named are rooted and fixed, while many true plants of lower grade are never rooted at all. The yeast plant, the _Algæ_ that swarm in our ponds, and the diatoms that crowd the waters, exemplify plants that are as free as animals; and many of them, besides, in their young state especially (e.g., the seaweeds), swim about freely in the water as if they were roving animalcules. On the second count, also, science gains the day; power of motion is no legitimate ground at all for distinguishing one living being as an animal, while absence of movement is similarly no reason for assuming that the fixed organism must of necessity be a plant. Then comes the microscopic evidence. What can this wonder glass do in the way of drawing boundary lines betwixt the living worlds? The reply again is disappointing to the doubter; for the microscope teaches us that the tissues of animals and plants are built upon kindred lines. We meet with cells and fibers in both. The cell is in each case the primitive expression of the whole organism. Beyond cells and fibers we see the wonderful living substance, _protoplasm_, which is alike to our senses in the two kingdoms, although, indeed, differing much here and there in the results of its work. On purely microscopic grounds, we cannot separate animals from plants. There is no justification for rigidly assuming that this is a plant or that an animal on account of anything the microscope can disclose. A still more important point in connection with this protoplasm question consists in the fact that as we go backward to the beginnings of life, both in animals and plants, we seem to approach nearer and nearer to an identity of substance which baffles the microscope with all its powers of discernment. Every