a long story, reaching over several centuries, but it can be briefly told.
Among the ancient Greeks observations directly upon animals and plants led to many facts of natural history. Aristotle (384-322 B. C.) is the best representative of the knowledge of his time about life. But after a few centuries the mind of mankind was turned away from nature. In due course of time, there was a complete arrest of inquiry into all things relating to the external world.
During this period, other branches of learning might make a little advance, but the knowledge of nature suffered the most, because we cannot know anything about natural phenomena without turning the mind outward and making direct observations upon the external world. Therefore, it was an epoch of great importance when men began again to observe, to use their eyes, and to turn to the great world of nature outside themselves. The men who started independent observations deserve much credit, for the authorities of both church and state were unfriendly to unbiased inquiry, and they went against every motive of self-interest in becoming pioneers in the new intellectual life. Vesalius (see Anatomy), Galileo and Descartes were among the reformers in the 16th century, and in the 17th century the work of Malpighi, Swammerdam and Leeuwenhoek is worthy of especial mention. Their great work consisted chiefly in this, that “they broke away from the thraldom of book-learning, and, relying alone upon their own eyes and their own judgment, won for man that which had been quite lost, the blessing of independent and unbiased observation.”
Thus awoke again the good spirit of inquiry and thereby the foundations of modern science were laid. When attention was turned to animals and plants, the first things noticed were, of course, the simplest and most obvious: external form, color, habits. This is the period in which the organism was studied as a whole, for each plant and animal is an organism. The naturalist of the time knew relatively little about the internal structure of animals and plants but many general facts about them. Linnæus and Ray represent this level of knowledge. For them, the study of nature consisted in observing and collecting widely, grouping or classifying animals and plants, learning about their habits, etc. Linnaeus made an epoch by introducing a method of naming plants and animals by giving to each two names—a generic and a specific name. This made knowledge more definite. He used Latin, which was the language of science, and, as his method was universally adopted, the same name came to apply to the same animal or plant in all countries. This directed the attention of naturalists to species or particular kinds, and thereby prepared the way for the discussion of the origin of species, which is the fundamental question in the doctrine of organic evolution.
But another advance was to come. The next natural step after the study of the organism as a whole was to think of its architecture or the way in which it is constructed. Men began, therefore, to observe the organs or parts that are united together to make up animals and plants. This was a study of internal structure. The construction of animals was studied very widely and they were compared with one another, so that there arose the new science of comparative anatomy (see Anatomy) of which Georges Cuvier (1769-1832) was the founder. Similar work was done for plants by Jussieu. But knowledge of nature was becoming so much extended that it was necessarily subdivided, and investigations into the uses of the organs were being made by physiologists like Haller and J. Müller (see Physiology) at the same time that structure was being studied by the anatomists.
The next step was based on the observation that organs are composed of simpler parts called tissues. Often several different kinds of tissue will unite into one organ; for example, the heart is not all muscle but connective tissue and nerves enter into it; the walls of the stomach also contain glands, muscles, nerves, connective tissue, etc., all united to form the one organ—the stomach. The leaf of a plant is not all one kind of tissue, but several different kinds enter into it. We are thus approaching step by step the finer structure of living beings. Bichat (1771-1802) studied the tissues extensively, and at the beginning of the 19th century laid the foundation of microscopic anatomy or histology.
The next step was based upon the perfection of the microscope. This instrument had been introduced, in a crude form, into natural history in the 17th century, and had opened a new world to naturalists. About 1840 great improvements were made in manufacturing the magnifying glasses for the microscope, and observers began gradually to see that the tissues are not the simplest parts of animals and plants, but that tissues are composed of very small units or particles, brought together and built into the tissues as bricks might be fitted together into a building. This idea took definite form about 1839-40, principally through the observations of two men, Schleiden and Schwann. The former was a botanist, and he came to the conclusion that all the parts of plants are built of little box-like compartments or cells. The latter, his friend, was an anatomist and zoölogist, who, from his studies with the microscope on animals, reached
