which is itself the result of past environments, and to set it up as a barrier against the influence of new environments.
Descending still deeper, scientific men have sought to explain the constitution of living things, their production, and the existence of the groups into which we find them divided. Hence three theories which have had different fortunes—the cellular theory, the doctrine of spontaneous generation, and transformism.
Schwann, applying to the animal organism Schleiden's discoveries in vegetal organisms, showed that the tissues are formed of primordial, i. e., irreducible, elements, called cells, though often these elements have no cavity and are simply rounded masses. The egg, which is the starting-point of all animal organisms, is at first merely a cell, and develops by producing within itself other cells, which are the primitive materials of the living being. All that the organism is comes ultimately from the cells, which are converted into living tissues. They adhere to one another end to end, and become flattened, or lengthened, or ramified; or they unite and form one common cavity, keeping their walls only at points where they are not in contact, thus forming tubes, or fibres, as, for example, in the histological elements of muscles and nerves.
Some authors have explained the production of cells on the hypothesis of a true spontaneous generation. According to them, cells are organized in a saline solution, the first step being the deposit of a nucleolus, around which there forms an envelope called the nucleus, and finally, at a greater distance, a second envelope, or cell-wall. But no actual experiment has ever been made on the production of cells in this way, and hitherto we have no knowledge of a cell being produced save from a cell. Of this famous theory so much yet remains, viz., that the cell, whatsoever its form and whatever modifications it may have received, is ever the basis of the vital phenomena.
The question as to how living bodies are produced gave rise, a few years ago, to discussions which have again brought to the surface a