Chapter XIV

, where it was pointed out that Weismann was not the originator of the idea, but he is nevertheless the one who has developed it the most extensively.

Complexity of the Germ-Plasm.--The germ-plasm has been molded for so many centuries by external circumstances that it has acquired an organization of great complexity. This appears from the following considerations: Protoplasm is impressionable; in fact, its most characteristic feature is that it responds to stimulation and modifies itself accordingly. These subtle changes occurring within the protoplasm affect its organization, and in the long run it is the summation of experiences that determines what the protoplasm shall be and how it will behave in development. Two masses of protoplasm differ in capabilities and potentialities according to the experiences through which they have passed, and no two will be absolutely identical. All the time the body was being evolved the protoplasm of the germinal elements was being molded and changed, and these elements therefore possess an inherited organization of great complexity.

When the body is built anew from the germinal elements, the derived qualities come into play, and the whole process is a succession of responses to stimulation. This is in a sense, on the part of the protoplasm, a repeating of its historical experience. In building the organism it does not go over the ground for the first time, but repeats the activities which it took centuries to acquire.

The evident complexity of the germ-plasm made it necessary for Weismann, in attempting to explain inheritance in detail, to assume the existence of distinct vital units within the protoplasm of the germinal elements. He has invented names for these particular units as biophors, the elementary vital units, and their combination into determinants, the latter being united into ids, idants, etc. The way in which he assumes the interactions of these units gives to his theory a highly speculative character. The conception of the complex organization of the germ-plasm which Weismann reached on theoretical grounds is now being established on the basis of observation (see