similar conclusions. This great discovery unites all animals and plants on a broad plane of similitude of structure. It is known as the cell theory, and has done much toward unifying the knowledge of animals and plants. This, taken in connection with the fact that all animals and plants arise from a single cell, has great meaning. The discovery that the egg of all animals is a single cell, shows that the starting-point is one of the single bricks of organic architecture, which by successive divisions gives rise to all the others that enter into the construction. Plants also proceed from a single cell. These are among the most remarkable facts in all nature. In its original form, however, the cell theory was very imperfect. Both Schleiden and Schwann supposed that the cell wall was important, and looked upon the cells as little box-like compartments. This had to be changed by later students, and the cell theory was reformed and modified into the protoplasm theory.
The progress, thus far, had brought out the facts that cells are joined to make tissues, the tissues to make organs, and organs to make the organism, but there was one further step to be taken to bring this line of advance to its proper goal. It was soon discovered that the cell wall of cells is not important, but that the jelly-like viscid fluid within is the substance that is actually alive. This substance is the seat of all life and is called protoplasm (see Protoplasm). We see that observations began on the outside and led by a series of steps to the true seat of life, just as a flight of stairs, uncovered by some good genii in the old fairy tales, led from the surface to a treasure cavern, but it took a good many years for naturalists to take each step. Max Schultze, in 1860, placed the ideas about protoplasm on a firm basis, and from that time dates modern biology, which is all about this living substance—-what it is like in its various manifestations, what it is doing, and what it has done in the world.
Therefore, a great deal of the work of the biologist is the study of this living substance at first hands. He can place under his microscope the simplest plants and animals, and if they are translucent enough to let the light through, he can see many things that are taking place within the protoplasm. One of the common organisms of great interest to biologists is the amœba—a simple microscopic particle of living jelly in which the processes of life are reduced to their simplest expression, and it meets all the requirements for observation. This organism is really an animal and a single cell, and, therefore, it lies near the bottom of the animal series.
By suitable experiment and observation it can be shown that the amœba, simple as it appears, is really very complex, on account of the powers and activities which it exhibits by virtue of being alive. A list or catalogue of its activities will be the same as those occurring in the various tissues and organs of higher animals. Therefore, we have in it the germ of all the activities of the higher creation. Its body is a little mass of protoplasm, and anything determined about it holds good for protoplasm. This substance is the only one in the world that is endowed with life, and biologists have come to the conclusion that it is practically identical in plants and animals, but at the same time exhibits a wide range of variations and differences, not in kind, but depending on the degree of perfection and specialization.
Protoplasm has properties which, taken together, distinguish it absolutely from every form of non-living matter. These are: (1) its chemical composition; (2) its power of waste and repair and of growth; (3) its power of reproduction. Other substances are simpler in composition than protoplasm, in fact, it is the most complex substance in the universe. Common chemical elements like carbon, hydrogen, oxygen, nitrogen, sulphur and phosphorus, enter into it, but they are combined in a very much more complex manner than in any other substance, and they are all present at the same time. Living matter is also continually undergoing a process of breaking down, by a sort of internal combustion, and making good the loss by the manufacture of new protoplasm out of the simpler food particles. It also has the power of growth, and “lastly, living matter not only thus repairs its own waste, but also gives rise, by reproduction, to new masses of living matter; which, becoming detached from the parent mass, enter forthwith upon an independent existence.”
“We may perceive how extraordinary these properties are by supposing a locomotive engine to possess like powers: to carry on a process of self-repair in order to compensate for wear; to grow and increase in size, detaching from itself at intervals pieces of brass or iron endowed with the power of growing up step by step into other locomotives capable of running themselves, and of reproducing new locomotives in their turn. Precisely these things are done by every living thing, and nothing like them takes place in the lifeless world.”—Sedgwick and Wilson, General Biology.
The higher animals, all of which are many-celled, may be looked upon as combinations of amœba-like elements, variously modified and built into the tissues. In passing from the condition of a single cell to that of many there has been not only an increase in the number of cells, but there has been also a physiological division of labor, so that particular groups of cells
