their environment which is caused by the presence of this or that toxin, and that this adaption is more readily achieved when it is accomplished by degrees—i.e. by a series of efforts.
Of course it is possible that skin-cells, muscle-cells, nerve-cells, leucocytes, &c. become adapted to changes in the environment through a process of Natural Selection, through the survival of the fittest among them, and that they do not really react to stimulation, but that the unfittest among them perish, leaving in each succeeding cell-generation the more and more fit to continue the race—i.e. that they do not transmit acquired traits any more than multicellular organisms do, but undergo evolution only through the accumulation of inborn variations. But this also is highly improbable, since the adaptive change is often accomplished in a time which is apparently too short to admit of such evolution among the cells; for instance, it is hardly possible that acquired immunity against small-pox is due to evolution among the phagocytes owing to the continued survival of their fittest. On the other hand, it is possible that this adaptive change in the cells is due to the survival of the fittest among their biaphors; that is, to the survival of the fittest among those hypothetical units which are supposed to stand in the same relation to the cell as the cell stands in relation to the multicellular organism.
