Manifestations of animal life are everywhere visible. They may be seen on mountain peaks, in desert plains, and by the seashores. Even the bleak arctic ice fields have their faunas. This extraordinary distribution of life has attracted attention since the dawn of history. Primitive man, by his often beautiful cave drawings, indicated that he studied intimately the wild life surrounding him. The basic facts of natural history were studied by the early peoples of the Near East. The Greeks prepared many books on natural history and anticipated modern evolutionary theories. The natural sciences, however, made slow progress until toward the end of the eighteenth century when Linnæus and Buffon began their great works. When the nineteenth century opened, the broader fields of nature were segregated, classified, and described. Linnæus took broad views regarding the principles of classification based upon general structure, and his work was enlarged and improved by Cuvier.
Buffon contributed suggestions regarding the probable mutability of species with respect to changes in environment, and improved on the old Greek evolutionary ideas by formulating a definite theory of the causes of mutability. He was an important agent in promoting the modern theories of evolution in zoölogy and botany, which have done more than anything else to augment our knowledge of terrestrial life.
The numerous scientific exploring expeditions in the eighteenth and nineteenth centuries collected an enormous amount of data regarding animals and animal life. Early in the nineteenth century this data was worked up and classified. It soon became apparent that the range of any given species of animal is strictly limited. A new science, that of the geographical distribution of life, was developed. This has been very fruitful in defining the true home areas of all species of animals, insects, birds, and fish, and locating their principal paths of migration.
The world has been divided into about a dozen terrestrial life regions, subregions and transitional regions. These have been mapped and described. The work of Dr. A. R. Wallace, in 1876, showed the comparative importance and extent of these life zones and their variable richness in zoölogical forms, the relationships of the species in different zones, and their degrees of isolation. The descriptions of these great geographical zones fill many interesting volumes and cover all the important forms of existing life.
The naturalists who studied particular zones, or classes of animals, frequently did extraordinary work. The bird studies in North America, recorded in a series of wonderful paintings by Audubon, and the studies of Fürbringer and other naturalists, are comparable with Wallace's great book on the Geographical Distribution of Animals, published in 1876.
The morphological researches of Parker, Huxley, Quatrefages, Owen, and others revolutionized many of the subdivisions of natural history and led to important discoveries in biology.
The effects of climate upon the development, migration, and decline of species and upon the extension and upbuilding of civilization have been minutely studied. Kropotkin showed that climatic changes in Asia drove the hordes of native tribes into Europe at early periods. They were forced to migrate on account of droughts leading to a food shortage. Many historical events have been shaped by climatic factors. Just as men who inhabit dry districts are usually nomads on account of their need of seeking new food supplies, so animals and insects are forced to migrate for a similar reason. The life changes wrought by disease epidemics under climatic influences have also been studied and have shed much light upon the origin and development of many organs and upon the habits of animals. Some of the chief inferences arising from investigations on the effects of climatic variations on life are that certain types of climate favor the development of certain animal species; certain climates have prevailed in historical times in centers where civilization flourished greatly. Therefore it may be presumed that definite climatic conditions are required for the specific development of each type of species and for each kind of civilization. Just as history shows that one of the many conditions of human progress has changed repeatedly from century to century on account of variations in climatic factors, so these stimuli have, from the earliest times, swayed and modified all classes of organic life. Climate serves to develop, retard, or extinguish animal characteristics, habits, and development. The study of the rôle of climate in modifying living conditions has disclosed data which throws much light on the philosophical problems surrounding organic life, its laws and progress.
The voyage of the Beagle in 1831, for a scientific cruise to South America, with Charles Darwin aboard as naturalist; that of the Ross Antarctic expedition in 1839, with Sir W. J. Hooker as botanist; that of the Rattlesnake for Australia and the South Seas in 1846, with T. H. Huxley as surgeon, resulted in the assembling of scientific data in natural history fields which, when classified and developed, revolutionized the natural sciences.
The work of the Challenger, in 1872, and many other memorable British scientific expeditions augmented and confirmed the data collected in the earlier explorations.
Harvey's explanation of the movement of the blood by the pumping pulsations of the heart quickened interest in biology. Mayer and Helmholtz, when chemists, had succeeded in artificially making urea and sugar and investigated living organisms from the viewpoint of mechanisms operated on the principle of the conservation of energy. They traced the manifold functions of the body to chemical and thermal energies developed by the destruction of food.