animals and plants still in use. Systema Naturæ, his great work, was first published in 1735, and reached the 12th edition in 1768. Buffon was a man of more philosophical mind than Linnæus. He also was a graceful writer and created a popular interest in natural history. He must not, however, be thought of as a mere popular writer; he opened new fields and led the way in matters of great importance. He paid much attention to the geographical distribution of animals, and for the first time treated the natural history of the various races of mankind scientifically. He reflected upon the relationship of animals, and is recognized as one of the early evolutionists. This particular line of thought was carried further by Lamarck (1744–1829) and St. Hilaire (1772–1844), the most noteworthy predecessors of Darwin.
The next phase of advance is marked by devotion to the study of structure. The first step in understanding a machine is to observe its parts, to know their relation and how they act on one another. That the animal machine is very complex will be admitted by the casual observer in respect to the higher animals, but there is a complexity about the simpler ones that is not generally appreciated. As Huxley said, when an oyster is swallowed few people realize that there passes the lips an organism more complicated in construction than a watch. The study of the structure of animals was carried on extensively and made comparative by Georges Cuvier (1769–1832) and his followers. They dissected and observed, and reduced the architecture of animals to a science. Thus arose comparative morphology which is now considered the first stage of zoölogical study. It will be readily seen that structure may be taken to refer only to the more obvious parts, visible to the unaided eye, or may be also extended to include the minute construction of animal tissues. Cuvier’s work in anatomy was mainly on the organs, but Bichat, his contemporary (1771–1801), pushed the analysis a step further and carefully examined the tissues. His influence on the progress of zoölogy was much greater than is generally appreciated. For it is a fact that has been gradually forced upon naturalists that the processes of life take place deep in the finer structure of animals and plants. Bichat did not reach the units of structure in his work, and studies in microscopic structure were carried to a deeper level before they reached this goal. This work was done for animal tissues in 1840 by Schwann (1810–81), and led to the establishment of the cell-theory (q. v.). In its original form the cell-theory was very imperfect; it was extended by the discovery that the cell is a globule of protoplasm containing a nucleus, and was molded into the protoplasm theory mainly through the work of Max Schultze in 1860. See Biology.
There is still another aspect of structural study that was brought out by observations on the development of animals. The structure of animals is so complicated that it is difficult to comprehend. There are, especially in higher animals, many rudimentary organs and traces of structures that do not conform to the life of the animal and to its position in the animal scale. Now, all animals begin their life in a microscopic rudiment — the egg, which in reality is a cell set free from the body of the parent. This cell by division gives rise to new cells which remain, connected, and form the many cells out of which the tissues are built. Thus animals start in a state of simplicity and grow to complexity. By observing the various stages through which they pass we get clues to the meaning of the rudimentary organs and to the relationships of animals, that would be completely lacking if we could not observe them in the process of becoming. Hence, embryology or the development (q. v.) of life becomes the key to understanding animal structures. This important branch of zoölogy was started in its modern phase by Wolff in 1759; but Von Baer (1792-1876) is designated the Father of Modern Embryology, because about 1827 he made this study a tracing of the history of the cell-layers.
The study of structure leads naturally to the question of use: What is the particular office or function of the organs, the tissues and the cells? This line of study is called physiology. It is to be understood that, while advances were being made in reference to the construction of animals, similar advances were in progress in reference to their physiology. William Harvey (1578–1657), Haller (1708–77) and Johannes Müller (1801–58) represent a succession of investigators in this line who pushed physiology forward. Müller, in particular, by his extraordinary industry and great insight both into anatomy and into physiology had great influence on the progress of zoölogy.
Zoölogy up to 1860 is the product of the concurrent growth of knowledge in regard to the structure of animals (morphology), their development (embryology) and their vital activities (physiology). Then an additional element was introduced which has illuminated the whole field. This was the doctrine of organic evolution, as set forth by Darwin in 1859. We must note in passing that this was not the beginning of the doctrine of organic evolution (q. v.); it was the particular form of Darwin’s explanation that led to its taking firm hold for the first time upon the minds of naturalists. From that time to the present the study of zoölogy has been dominated
