==The Discovery of Cells.==--It is difficult for us to-day to have any adequate idea of the wonderful flood of light that was thrown upon scientific and philosophical study by the discoveries which are grouped around the terms cells and protoplasm. Cells and protoplasm have become so thoroughly a part of modern biology that we can hardly picture to ourselves the vagueness of knowledge before these facts were recognized. Perhaps a somewhat crude comparison will illustrate the relation which the discovery of cells had to the study of life. Imagine for a moment, some intelligent being located on the moon and trying to study the phenomena on the earth's surface. Suppose that he is provided with a telescope sufficiently powerful to disclose moderately large objects on the earth, but not smaller ones. He would see cities in various parts of the world with wide differences in appearance, size, and shape. He would see railroad trains on the earth rushing to and fro. He would see new cities arising and old ones increasing in size, and we may imagine him speculating as to their method of origin and the reasons why they adopt this or that shape. But in spite of his most acute observations and his most ingenious speculation, he could never understand the real significance of the cities, since he is not acquainted with the actual living unit. Imagine now, if you will, that this supramundane observer invents a telescope which enables him to perceive more minute objects and thus discovers human beings. What a complete revolution this would make in his knowledge of mundane affairs! We can imagine how rapidly discovery would follow discovery; how it would be found that it was the human beings that build the houses, construct and run the railroads, and control the growth of the cities according to their fancy; and, lastly, how it would be learned that it is the human being alone that grows and multiplies and that all else is the result of his activities. Such a supramundane observer would find himself entering into a new era, in which all his previous knowledge would sink into oblivion. Something of this same sort of revolution was inaugurated in the study of living things by the discovery of cells and protoplasms. Animals and plants had been studied for centuries and many accurate and painstaking observations had been made upon them. Monumental masses of evidence had been collected bearing upon their shapes, sizes, distribution, and relations. Anatomy had long occupied the attention of naturalists, and the general structure of animals and plants was already well known. But the discoveries starting in the fourth decade of the century by disclosing the unity of activity changed the aspect of biological science.