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tracts. It recognizes its food even at a microscopic distance; it appears therefore to feel and perceive. Perhaps we might say that it has a mind and will of its own. It is safer to say that it is irritable, that is, it reacts to stimuli too feeble to be regarded as tho cause of its reaction. It engulfs microscopic plants, and digests them in the internal protoplasm by the aid of an acid secretion. It breathes oxygen, and excretes carbonic acid
and urea, through its whole body surface. Its mode of gaining the energy which it manifests is therefore apparently like our own, by combustion of food material.
1. AMOEBA PROTEUS. HERTWIG, FROM LEIDY. ek, ectosarc; en, endosarc; N, food particles; n, nucleus; cv, contractile vesicle.
It grows and reaches a certain size, then constricts itself in the middle and divides into two. The old amoeba has divided into two young ones, and there is no parent left to die, and death, except by violence, does not occur. But this absence of death in other rather distant relatives of the amoeba, and probably in the amoeba itself, holds true only provided that, after a series of self-divisions, reproduction takes place after another mode. Two rather small and weak individuals fuse together in one animal of renewed vigor, which soon divides into two
larger and stronger descendants. We have here evidently a process corresponding to the fertilization of the egg in higher animals; yet there is no egg, spermatozoon, or sex.
It is a little mass of protoplasm containing a nucleus, and corresponds, therefore, to one of the cells, most closely to the egg-cell or spermatozoon of higher animals. If every living being is descended from a single cell, the fertilized egg, it is not hard to believe that all higher animals are descended from an ancestor having the general structure or lack of structure of the amœba.
But is the amoeba really structureless? Probably it has an exceedingly complex structure, but our microscopes and technique are still too imperfect to show more than traces of it. Says Hertwig: "Protoplasm is not a single chemical substance, however complicated, but a mixture of many substances, which we must picture to ourselves as finest particles united in a wonderfully complicated structure." Truly protoplasm is, to borrow Mephistopheles' expression concerning blood, a "quite peculiar juice." And the complexity of the nucleus is far more evident than that of the protoplasm. Is protoplasm itself the result of a long development? If so, out of what and how did it develop? We cannot even guess. But the beginning of life may, apparently must, have been indefinitely farther back than the simplest now existing form. The study of the amoeba cannot fail to raise a host of questions in the mind of any thoughtful man.
As we have here the animal reduced, so to speak, to lowest terms, it may be well to examine a little more closely into its physiology and compare it briefly with
The amoeba eats food as we do, but the food is digested directly in the internal protoplasm instead of in a stomach; and once digested it diffuses to all parts of the cell; here it is built up into compounds of a more complex structure, and forms an integral part of the animal body. The dead food particle has been transformed into living protoplasm, the continually repeated miracle of life. But it does not remain long in this condition. In contact with the oxygen from the air it is soon oxidized, burned up to furnish the energy necessary for the motion and irritability of the body. We are all of us low-temperature engines. The digestive function exists in all animals merely to bring the food into a soluble, diffusible form, so that it can pass to all parts of the body and be used for fuel or growth. In our body a circulatory system is necessary to carry food and oxygen to the cells and to remove their waste. For most of our cells lie at a distance from the stomach, lungs, and kidney. But in a small animal the circulatory system is often unnecessary and fails. Breathing and excretion take place through the whole surface of the body. The body of the frog is devoid of scales, so that the blood is separated from the surrounding water only by a thin membrane, and it breathes and excretes to a certain extent in the same way.
But another factor has to be considered. If we double each dimension of our amoeba, we shall increase its surface four times, its mass eight-fold. Now the power of absorbing oxygen and excreting waste is evidently proportional to the excretory and respiratory surface, and much the same is true of digestion. But the amount of oxygen required, and of waste to be removed is proportional to the mass; for every par
ticle of protoplasm requires food and oxygen, and produces waste. The particles of protoplasm in our new, larger amoeba can therefore receive only half as much oxygen as before, and rid themselves of their waste only half as fast. There is danger of what in our bodies would be called suffocation and blood-poisoning. The amoeba having attained a certain size meets this emergency by dividing into two small individuals, the division is a physical adaptation. But the many-celled animal cannot do this; it must keep its cells together. It gains the additional surface by folding and plaiting. And the complicated internal structure of higher animals is in its last analysis such a folding and plaiting in order to maintain the proper ratio between the exposed surface of the cells and their mass. And each cell in our bodies lives in one sense its own individual life, only bathed in the lymph and receiving from it its food and oxygen instead of taking it from the water.
But in another sense the cells of our body live an entirely different life, for they form a community. Division of labor has taken place between them, they are interdependent, correlated with one another, subject therefore to the laws of the whole community or organism. There are many respects in which it is impossible to compare Robinson Crusoe with a workman in a huge watch factory; yet they are both men.
Both the amoeba and we live in the closest relation to our environment, and conformity to it is evidently necessary life has been defined as the adjustment of internal relations to external conditions. We continually take food, use it for energy and growth, and return the simpler waste compounds. We are all of us, as
Professor Huxley has said, "whirlpools on the surface of Nature;" when the whirl of exchange of particles ceases we die. We have seen that the fusion of two amœbæ results in a new rejuvenated individual. Why is a mixture of two protoplasms better than one? We can frame hypotheses; we know nothing about it. What of the mind of the amoeba? A host of questions throng upon us and we can answer no one of them. All the great questions concerning life confront us here in the lowest term of the animal series, and appear as insoluble as in the highest.
Our second ancestral form is also a fresh-water animal, the hydra. This is a little, vase-shaped animal, which usually lives attached to grass-stems or sticks, but has the power to free itself and hang on the surface of the water or to slowly creep on the bottom. The mouth is at the top of the vase, and the simple, undivided cavity within the vase is the digestive cavity. Around the mouth is a ring of from four to ten hollow tentacles, whose cavities communicate freely underneath with the digestive cavity. Not only is food taken in at the mouth, but indigestible material is thrown out here. The animal may thus be compared to a nearly cylindrical sack with a circle of tubes attached to it above. The body consists of two layers of cells, the ectoderm on the outside and the entoderm lining the digestive cavity. Between these two is a structureless, elastic membrane, which tends to keep the body moderately expanded.
The food is captured by the tentacles; but digestion takes place only partially in the digestive cavity, for each surrounding cell engulfs small particles of food and digests them within itself. The entodermal cells