The New Student's Reference Work/Zoölogy
Zoöl′ogy, the science of animal life. The scope of this subject broadened greatly in the latter half of the 19th century, and it came to include questions that might not at first sight appear to have a direct connection with zoölogy. For example, certain discoveries (those of Von Lenhossék) as to the nervous system of the earthworm opened the way to generalizations regarding the nervous system of higher animals, including that of the human body. Again, the studies of zoölogists on cells are helping to solve broad questions of development and heredity. Thus zoölogy comes into direct relation with the work of the anatomist, the physiologist and the psychologist. This relation is so close that medical schools are asking for a preparation in zoölogy as an introduction to medical studies. One impression that has gained wide currency should be corrected: viz., that the chief business of the zoölogist is to know the different kinds of animals and be able to name them. That is zoölogy of the past, but it has been replaced by a better kind. Attention has been directed from the external appearances of animals, as shape, color, differences in horns, hoofs etc., to their internal structure and life processes. An attempt is being made to analyze their vital activities and to determine their position in the history of the universe. An examination of the textbooks of a generation ago will show that emphasis was unifomly placed on the classification of animals. In this period, also, the collector for museums and the hunter naturalist were dominant types of zoölogists. It is quite true that students of zoölogy should not be drawn away from the fields, forests and streams; nevertheless, that which is best and strongest in modern zoölogy is being worked out in the laboratory by experiments and observations on a few forms. These studies, especially in the last half of the 19th century, are leading to large generalizations regarding the science of life, which should become known to all people of liberal culture.
Zoölogy as a science aims to give a picture of animal life on the globe; to consider its structure; to analyze its activities; to account for it by tracing its history in general and in detail; and to consider its relation to nature at large. The history of biology includes that of zoölogy, and the article on biology should be consulted, where a number of general considerations are entered upon that need not be repeated here.
The new way of looking at the animal world did not come all at once, but has been the result of growth. Knowledge regarding animal life has advanced by a series of steps, and its progress can be divided into a number of periods. The first of these is the period extending from the earliest writings about animals to the renaissance of science in the 16th century. Zoölogical study was cultivated among ancient peoples and reached its highest development in that period in the work of Aristotle, the Hellenic philosopher-naturalist, who lived from B. C. 384 to B. C. 322. He was a man of truly scientific mind and made many original observations on the structure and development of animal life. With him classification was based upon structure, but was of secondary importance. His successors did not reach the level of the great Father of Natural History, as he has been called. Their work more and more took the line of systematic classification of animals, and lost in observations on structure and development. Pliny the Elder (A. D. 23–79) is the next prominent naturalist of antiquity. That Roman general and teacher has been unduly praised as a zoölogist, for he merely was a poor compiler, who mixed borrowed facts, fancy and fabulous stories. He replaced the natural classification of Aristotle by a highly artificial arrangement of animals, according to their place of abode on land or in water or in air. It is noteworthy that nothing of importance was added to the work of Aristotle till the awakening of observation in the 16th century.
In the intervening period the intellectual life of mankind took a retrograde movement. Observations and inquiry into nature were discontinued and, in fact, were discouraged by intellectual leaders. This led to superstition in reference to the natural universe. But an awakening came, and it was an epoch of unusual importance when men began once more to observe and experiment and so to lay the foundations of modern science. The new intellectual movement was led in the 16th century by Galileo, Descartes and Vesalius. The work of Vesalius (1514–64) in anatomy (q. v.) had great influence upon the development of zoölogy. Progress in anatomy helped zoölogy by giving a knowledge of structure. At first structural studies mainly were on the higher animals nearest man. Afterwards they were extended to other forms, including the invertebrates, and were made broadly comparative.
At the revival of learning the influence of Aristotle was mainly on the systematic classification of animals. His works were translated and extended by Wotton (1492–1555), Gesner (1516–65), Aldrovandi (1522–1605) and others who produced ponderous Latin treatises on natural history. These men and their successors for about two hundred years have been called the encyclopedists. The work of Gesner was of fine quality, and marks the beginning of careful observation and description of animals.
Passing over the notable work of naturalists like Malpighi (1628–94), Leeuwenhoek (1632–1723) and Swammerdam (1637–80) in the 17th century, who worked on minute anatomy and brought the microscope into use, we come to the period characterized by the work of Ray (1628–1705), Linnæus (1707–78) and Buffon (1707–88). Ray laid the foundations upon which Linnæus built. The name of the latter probably is more widely known than that of any other naturalist. He was a man of prodigious industry, filled with a zeal for collecting and naming natural objects. He greatly extended the number of known animals and plants, but did little toward deepening the knowledge of animal life. His chief service was in systematizing the groups and introducing the method of naming 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 by the idea of evolution, and animals and plants have been studied broadly, — in the light of their ancestral history. Progress is dependent still upon advance in anatomy, physiology and embryology, but the point of view from which the facts are considered has been changed.
We have now traced the rise of zoölogy in its principal phases down to the beginning of its especially modern aspect. The study of animal life is so great a field that naturally it has a number of divisions; it remains, therefore, to mention in a summary way its chief departments. That part of the subject which embraces the study of animal structure is Morphology. This word is generally used in a broad sense to mean something more than Anatomy, which is one division of morphology. The study of microscopic anatomy is Histology. It is not a different kind of study, and is separated from anatomy merely as a matter of convenience. The investigation of stages in animal development is Embryology. These divisions make up Structural Zoölogy. The general description and classification of animals is Systematic Zoölogy. The modern name for classification is Taxonomy. The Geographical Distribution of animals has been widely studied and elevated to the rank of a special department. Another set of studies concerning the lineage of all animals (Evolution) has led to broad considerations of the relation of living animals to their surroundings, to the universe and to man, and has given rise to Philosophical Zoölogy. Standing co-ordinate with the great field of morphology is Physiology, which, broadly speaking, concerns itself with the vital processes of all living organisms. General Physiology clearly is a department of zoölogy. Many experiments are being made to determine the responses and adaptations of animals to stimulations and to changes in the medium in which they live. This has opened the field of Experimental Morphology. Studies of the mental powers and the mental phenomena in animals lead to Animal Psychology, which may fairly be considered a part of zoölogy. A division called Ætiology is often recognized, having as its object the investigation of the causes of zoölogical phenomena, but it need not be assigned an independent rank, as the study of all phenomena has the discovery of their causes as an ultimate object.
The animal world is too extensive to attempt to give an outline view of it here, and reference must be made to textbooks for further consideration. Fuller accounts of some of the subjects touched upon in this article will be found under Anatomy, Development of Animal Life, Evolution, Cell-Doctrine, Physiology, Protoplasm and Spontaneous Generation
See, also, articles on such zoölogists as Aristotle, Cuvier, Darwin, Mivart, Romanes, Wallace, Weismann and others.
Among the best books for the general reader and student of zoölogy are the following: Agassiz’s Methods of Study in Natural History; Huxley’s The Crayfish, An Introduction to the Study of Zoölogy; Hertwig’s General Principles of Zoölogy; Lankester’s Zoölogy in The Encyclopædia Britannica; Sedgwick and Wilson’s General Biology; Parker’s Elementary Biology; Thompson’s The Science of Life, The Study of Animal Life and Outlines of Zoölogy; Verworn’s General Physiology; Jordan and Kellogg’s Animal Life; Bell’s Comparative Anatomy and Physiology; Parker and Haswell’s Textbook of Zoölogy and Manual of Zoölogy; Boas’ Textbook of Zoölogy; Brook’s Handbook of Invertebrate Zoölogy; McMurrich’s Invertebrate Morphology; Shipley and McBride’s Textbook of Zoölogy; Weidersheim’s Comparative Anatomy of Vertebrates; Lang’s Comparative Anatomy; Romanes’ Darwin and after Darwin; Osborn’s From the Greeks to Darwin; Balfour’s Comparative Embryology; Marshall’s Vertebrate Embryology; Hertwig’s Textbook of Embryology; Korschelt and Heider’s Invertebrate Embryology; Wallace’s Geographical Distribution of Animals and Malay Archipelago; Bates’ Naturalist on the Amazons; Heilprin’s Geographical and Geological Distribution of Animals; and The Standard Natural History, edited by Kingsley. For books on birds, butterflies, insects etc. see those topics. Among the best smaller zoölogies for secondary schools are those of Colton, Davenport, Kellogg, Needham and Packard.