Part 2 — Notes to Chapter XXX.
Reading edition
It appears probable that calcium is not always needed by living cells, and may not enter into the composition of protoplasm; on the other hand traces of iron are perhaps necessary.
The criticisms and summary of facts on which the hypothesis regarding protoplasm here adopted is based are developed at length in Kassowitz, Allgemeine Biologie, Wien, 1899, B. I. and II., where the collected literature may be found, and the reader introduced to the huge mass of controversial writings put forward since Darwin and associated with the names of Weismann and others.
It will probably be noticed that I have employed the term molecular unit of protoplasm, and have not discussed the question of organised structure in the latter: this is because it seems clear to me that living protoplasm as such does not possess "organised structure" in the true sense of that term—it is, rather, busy preparing and making "organised structure," and a molecular constitution would have to be ascribed to all "physiological units" of the nature of micellæ, pangens, ids, etc., as truly as to the structural units of a starch-grain or cell-wall, or even of a crystal. In this connection, the student will find the necessary points of view put forward in Pfeffer, Physiology, pp. 37-83.