so as again to form unicellular organisms. But each unicellular organism would in general inherit the peculiarities, and repeat the life-history of its grandparent-cell, by dividing into two adherent cells. A race of two-celled organisms would thus be established. We may fairly believe that in time a second variation, which also proved fortunate, occurred, whereby the four grand-daughter cells remained adherent until reproduction; and afterwards other variations of the same nature, till an organism was at length evolved which consisted of a multitude of cells adherent together for the common benefit—the morula. The component cells would all be alike, each would perform all the functions of life, assimilation, locomotion, &c.; and when reproduction (i.e. reproduction of the whole organism) took place the whole would break up into single cells, every one of which, by repeating one by one the variations of its ancestors, would build up, step by step, a multicellular organism similar to that from which its parent cell was derived; the steps of the ontogeny or development of the individual thus following those of the phylogeny, or evolution of the race.
The cells of such a simple multicellular organism, though similar in kind, would yet necessarily differ as regards position, some being internal and some external, and therefore it would obviously be an advantage (1) if the organism assumed such a shape as would enable all its constituent cells to perform their common functions to the greatest advantage; or (2) if among the cells a division of labour took place by virtue of which the internal and external cells performed functions different in kind; or (3) if the two variations were combined so that changes in the shape of the organism were accompanied by differentiations in the functions of the cells, some cells taking on one function, e.g. food-getting or locomotion, some another, e.g. digestion or
