The Organism.—In the time of Linnæus the attention of naturalists was mainly given to the organism as a whole. Plants and animals were considered from the standpoint of the organism—the external features were largely dealt with, the habitat, the color, and the general appearance—features which characterize the organism as a whole. Linnæus and Jussieu represent this phase of the work, and Buffon the higher type of it. Modern studies in this line are like addition to the Systema Naturæ.
Organs.—The first distinct advance came in investigating animals and plants according to their structure. Instead of the complete organism, the organs of which it is composed became the chief subject of analysis. The organism was dissected, the organs were examined broadly, and those of one kind of animal and plant compared with another. This kind of comparative study centered in Cuvier, who, in the early part of the nineteenth century, founded the science of comparative anatomy of animals, and in Hofmeister, who examined the structure of plants on a basis of broad comparison.
Tissues.—Bichat, the famous contemporary of Cuvier, essayed a deeper level of analysis in directing attention to the tissues that are combined to make up the organs. He distinguished twenty-one kinds of tissues by combinations of which the organs are composed. This step laid the foundation for the science of histology, or minute anatomy. Bichat called it general anatomy (Anatomie Générale, 1801).
Cells.—Before long it was shown that tissues are not the real units of structure, but that they are composed of microscopic elements called cells. This level of analysis was not reached until magnifying-lenses were greatly improved—it was a product of a closer scrutiny of nature with improved instruments. The foundation of the work, especially for plants, had been laid by Leeuwenhoek, Malpighi, and Grew. But when the broad generalization, that all the tissues of animals and plants are composed of cells, was given to the world by Schleiden and Schwann, in 1838-39, the entire organization of living forms took on a new aspect. This was progress in understanding the morphology of animals and plants.
Protoplasm.—With improved microscopes and attention directed to cells, it was not long before the discovery was made that the cells as units of structure contain protoplasm. That this substance is similar in plants and animals and is the seat of all vital activity was determined chiefly by the researches of Max Schultze, published in 1861. Thus step by step, from 1758, the date of the tenth edition of the Systema Naturæ, to 1861, there was a progress on the morphological side, passing from the organism as a whole to organs, to tissues, to cells, and finally to protoplasm, the study of which in all its phases is the chief pursuit of biologists.
The physiological side had a parallel development. In the period of Linnæus, the physiology of the organism was investigated by Haller and his school; following him the physiology of organs and tissues was advanced by J. Müller, Bichat, and others. Later, Virchow investigated the physiology of cells, and Claude Bernard the chemical activities of protoplasm.
This set forth in outline will be amplified in the following chapters.