and limited, nutrition is no longer possible in the old simple way and response to stimuli comes only in certain channels. In return for these sacrifices we may say, metaphorically, that the action of natural selection is withdrawn from within the commune. Unfitness of a given cell or group of cells can be eliminated only through its effect upon the whole organism. The latter is less sensitive to the vagaries of a single cell than is the organism of which the single cell is the whole. It would seem, therefore, that there is now allowed a greater range of variability for the individual cells, and perhaps, therefore, an increased richness of the material to be selected from. Variations, moreover, which were not immediately favourable would now have a chance of surviving.
Looked at in this way, multicellularity presents itself as an escape from the rigour of natural selection, which for the unicellular organism had narrowed {19} competition to so desperate a struggle that any variation outside the straitest limits was fatal, for even though it might be favourable in one respect, it would, in so small a kingdom, involve a loss in another. The only way, therefore,