It is far from correct to say, as is often done, that the cell-wall, or cell-membrane, belongs ãto the passive products of protoplasm rather than to the living cell itselfã; or to say that in the animal cell, the cell-wall, because it is ãslightly developed,ã is relatively unimportant compared with the important role which it assumes in plants. On the contrary, it is quite certain that, whether visibly differentiated into a semi-permeable membrane, or merely constituted by a liquid film, the surface of the cell is the seat of {199} important forces, capillary and electrical, which play an essential part in the dynamics of the cell. Even in the thickened, largely solidified cellulose wall of the plant-cell, apart from the mechanical resistances which it affords, the osmotic forces developed in connection with it are of essential importance.
But if the cell acts, after this fashion, as a whole, each part interacting of necessity with the rest, the same is certainly true of the entire multicellular organism: as Schwann said of old, in very precise and adequate words, ãthe whole organism subsists only by means of the reciprocal action of the single elementary parts58.ã
As Wilson says again, ãthe physiological autonomy of the individual cell falls into the backgroundã...ãand the apparently composite character which the multicellular organism may exhibit is owing to a secondary distribution of its energies among local centres of action59.ã
It is here that the homology breaks down which is so often drawn, and overdrawn, between the unicellular organism and the individual cell of the metazoon60.
Whitman, Adam Sedgwick61, and others have lost no opportunity of warning us against a too literal acceptation of the cell-theory, against the view that the multicellular organism is a colony (or, as Haeckel called it (in the case of the plant), a ãrepublicã) of independent units of life62. As Goethe said long ago, ãDas lebendige ist zwar in Elemente {200} zerlegt, aber man kann es aus diesen nicht wieder zusammenstellen und beleben;ã the dictum of