Bacteria and Nitrates.--One further illustration of the connection between bacteria and practical affairs may be mentioned. It is well known that animals are dependent upon plants, and that plants in the manufacture of protoplasm make use of certain nitrites and nitrates which they obtain from the soil. Now, the source of these nitrites and nitrates is very interesting. In animals the final products of broken-down protoplasm are carbon dioxide, water, and a nitrogenous substance called urea. These products are called excretory products. The animal machine is unable to utilize the energy which exists in the form of potential energy in these substances, and they are removed from the body. The history of nitrogenous substance is the one which at present interests us the most. Entering the soil, it is there acted upon by bacteria residing in the soil, these bacteria possessing the power of making use of the lowest residuum of energy left in the nitrogenous substance. They cause the nitrogen and the hydrogen to unite with oxygen in such a way that there are produced nitrous and nitric acids, and from these two acids, through chemical action, result the nitrites and the nitrates. These substances are then utilized by the plant in the manufacture of protoplasm, and the plant is fed upon by animal organisms, so that a direct relationship is established between these lower forms of life and the higher plant and animal series; a relationship that is not only interesting, but that helps to throw an important side-light upon the general nature of vital activities, their kind and their reach. In addition to the soil bacteria mentioned above, there are others that form association with the rootlets of certain plants and possess the power of fixing free nitrogen from the air. The nitrifying bacteria, are, of course, of great importance to the farmer and the agriculturist. It is not our purpose, however, to trace the different phases of the subject of bacteriology to their conclusions, but rather to give a picture of the historical development of this subject as related to the broader one of general biology. CHAPTER XIV HEREDITY AND GERMINAL CONTINUITY--MENDEL, GALTON, WEISMANN It is a matter of common observation that in the living world like tends to produce like. The offspring of plants, as well as of animals, resembles the parent, and among all organisms endowed with mind, the mental as well as the physical qualities are inherited. This is a simple statement of the fact of heredity, but the scientific study of inheritance involves deep-seated biological questions that emerged late in the nineteenth century, and the subject is still in its infancy.