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. These valuable discoveries were augmented by Schleiden and Schwann, showing that all organisms are built up of living cells. The offices performed within cells by colloids and solutions, and in the nerves by electric movements, were traced. Investigations into the most minute forms of animal life also furnished startling results. Schwann found, in 1838, that fermenting yeast consists of living vegetable cells, and that organic putrefaction is caused by the activities of such cells. Louis Pasteur (1822-1895) demonstrated that the presence of bacteria in any animal is always due to the entrance of bacteria and microbes from the outside, or by means favoring the abnormal increase of existing germs. He also showed by experiments that diseases like chicken cholera, phylloxera, or the silkworm disease are caused by particular microbes. These discoveries led to the tracing of many common diseases to their special living germs. While these impressive additions to scientific knowledge were being made, other naturalists were studying the instinctive emotional and intelligent behavior and psychology of animals, both singly and in herds. Animals and insects were found to display signs of intelligence, sometimes of a high order; to live socially, in many cases; and to play and court with emotional attributes. Throughout the animal kingdom, until man is reached, animals are guided in their activities by self and racial preservation. Play was found to be a fruitful factor in animal education, even in minute insects. The behavior of any animal does not stand alone, but is related to that of others. Animals which hunt, or are hunted, combatants, rivals, mates, and enemies, react upon one another.