MODIFICATION OF THE GERM-PLASM
Every living creature was at some stage of its life nothing more than a 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