Page:A Text-book of Animal Physiology.djvu/33

Rh alone can beget protoplasm; cell begets cell. Omne animal (anima, life) ex ovo applies with a wide interpretation to all living forms.

From what has been said it will appear that life is a condition of ceaseless change. Many of the movements of the protoplasm composing the cell-units of which living beings are made are visible under the microscope; their united effects are open to common observation—as, for example, in the movements of animals giving rise to locomotion we have the joint result of the movements of the protoplasm composing millions of muscle-cells. But, beyond the powers of any microscope that has been or probably ever will be invented, there are molecular movements, ceaseless as the flow of time itself. All the processes which make up the life-history of organisms involve this molecular motion. The ebb and flow of the tide may symbolize the influx and efflux of the things that belong to the inanimate world, into and out of the things that live.

It follows from this essential instability in living forms that life must involve a constant struggle against forces that tend to destroy it; at best this contest is maintained successfully for but a few years in all the highest grades of being. So long as a certain equilibrium can be maintained, so long may life continue and no longer.

The truths stated above will be illustrated in the simpler forms of plants and animals in the ensuing pages, and will become clearer as each chapter of this work is perused. They form the fundamental laws of general biology, and may be formulated as follows:

1. Living matter or protoplasm is characterized by its chemical composition, being made up of carbon, hydrogen, oxygen, and nitrogen, arranged into a very complex molecule.

2. Its universal and constant waste and its repair by interstitial formation of new matter similar to the old.

3. Its power to give rise to new forms similar to the parent ones by a process of division.

4. Its manifestation of periodic changes constituting development, decay, and death.

Though there is little in relation to living beings which may not be appropriately set down under zoölogy or botany, it tends to breadth to have a science of general biology which deals with the properties of things simply as living, irrespective very much as to whether they belong to the realm of animals or plants. The relation of the sciences which may be regarded