single cell. It is generally known that human beings result from the
union of an egg-cell and a sperm-cell, but it is not so universally
understood that these germ-cells are part of a continuous stream of
germ-plasm which has been in existence ever since the appearance of life on the globe, and which is destined to continue in existence as long as life remains on the globe.
The corollaries of this fact are of great importance. Some of them will
be considered in this chapter.
Early investigators tended naturally to look on the germ-cells as a product of the body. Being supposedly products of the body, it was natural to think that they would in some measure reproduce the character of the body which created them; and Darwin elaborated an ingenious hypothesis to explain how the various characters could be represented in the germ-cell. The idea held by him, in common with most other thinkers of his period, is still held more or less unconsciously by those who have not given particular attention to the subject. Generation is conceived as a direct chain: the body produces the germ cell which produces another body which in turn produces another germ-cell, and so on.
But a generation ago this idea fell under suspicion. August Weismann,
professor of zooelogy in the University of Freiburg, Germany, made himself the champion of the new idea, about 1885, and developed it so effectively that it is now a part of the creed of nearly every biologist.
Weismann caused a general abandonment of the idea that the germ-cell ism produced by the body in each generation, and popularized the conception of the germ-cell as a product of a stream of undifferentiated germ-plasm, not only continuous but (potentially at least) immortal.
The body does not produce the germ-cells, he pointed out; instead, the germ-cells produce the body. The basis of this theory can best be understood by a brief consideration of the reproduction of very simple organisms. "Death is the end of life," is the belief of many other persons than the Lotus Eaters. It is commonly supposed that everything which lives must eventually die. But study of a one-celled animal, an Infusorian, for example, reveals that when it reaches a certain age it pinches in two, and each half becomes an Infusorian in all appearance identical with the original cell. Has the parent cell then died? It may rather be said to survive, in two parts. Each of these daughter cells will in turn go
through the same process of reproduction by simple fission, and the process will be continued in their descendants. The Infusorian can be
called potentially immortal, because of this method of reproduction.
The immortality, as Weismann pointed out, is not of the kind attributedm by the Greeks to their gods, who could not die because no wound could destroy them. On the contrary, the Infusorian is extremely fragile, and is dying by millions at every instant; but if circumstances are favorable, it can live on; it is not inevitably doomed to die sooner
or later, as is Man. "It dies from accident often, from old age never."
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