To man, then, is a vast and a minute. Our minds ache at the contemplation of astronomical immensities, and we are apt to see the boundless only in prodigious masses, countless numbers, and immeasurable spaces. The Creative Mind knows no such limitations; and the microscope shows us that, whether the field of nature's operation be what to our apprehension is great or small, there is no limit to the exhibition of marvellous skill. If the "undevout astronomer" be "mad," the undevout microscopist must be still more so, for if the matter be judged by human sense, the skill is greater as the operation is more minute; and not the sun itself, nor the central orb round which he revolves, with all his attendant worlds, can furnish sublimer objects of contemplation, than the miraculous assemblage of forces which make up the life of the smallest creature that the microscope reveals. There is an irresistible charm in the effort to trace _beginnings_ in nature. We know that we can never succeed; that each discovery, which conducts back towards some elementary law or principle, only indicates how much still lies behind it: but the geologist nevertheless loves to search out the first or oldest traces of life upon our globe; and so the microscopist delights to view the simplest exhibitions of structures and faculties, which reach their completion in the frame and mind of man. That one great plan runs through the whole universe is now an universally accepted truth, and when applied to physiology and natural history, it leads to most important results. The researches of recent philosophers have shown us that nature cannot be understood by studying the parts of animals with reference merely to their utility in the economy of the creature to which they belong. We do, indeed, find an admirable correspondence between structures and the services they perform; but every object in creation, and every part of it, is in harmonious relation to some grand design, and exhibits a conformity to some general mode of operation, or some general disposition and direction of forces, which secures the existence of the individual or the species, and at the same time works out the most majestic schemes. Microscopic researches, such as are within the reach of millions, offer many of the most beautiful illustrations of these truths; and although the following pages are confined to such objects as are easily obtainable from ponds, and relate almost exclusively to the Infusoria, the Rotifers, the Polyps, and the Polyzoa, it is hoped that they will assist in associating a few of the highly suggestive reasonings of science, with one of the most pleasurable recreations that human ingenuity has devised.