the germ-plasm being always set aside, undifferentiated, to be handed on to the next generation.
In the light of this example, one can better understand the definition of germ-plasm as "that part of the substance of the parents which does not die with them, but perpetuates itself in their offspring." By bringing his imagination into play, the reader will realize that there is no limit to the backward continuity of this germ-plasm in the threadworm. Granted that each species has arisen by evolution from some other, this germ-cell which is observed in the body of the threadworm, must be regarded as part of what may well be called a stream of germ-plasm, that reaches back to the beginning of life in the world. It will be equally evident that these is no foreordained limit to the forward extension of the stream. It will continue in some branch, as long as there are any threadworms or descendants of threadworms in the world.
The reader may well express doubt as to whether what has been demonstrated for the threadworm can be demonstrated for the higher animals, including man. It must be admitted that in many of these animals conditions are too unfavorable, and the process of embryology too complicated, or too difficult to observe, to permit as distinct a demonstration of this continuity of the germ-plasm, wherever it is sought. But it has been demonstrated in a great many animals; no facts which impair the theory have been discovered; and biologists therefore feel perfectly justified in generalizing and declaring the continuity of germ-plasm to be a law of the world of living things.
Focusing attention on its application to man, one sees that the race must represent an immense network of lines of descent, running back through a vast number of different forms of gradually diminishing specialization, until it comes to a point where all its threads merge in one knot—the single cell with which