38 ORGANIC EVOLUTION — PHYSICAL 



SO as again to form unicellular organisms. But each 

 unicellular organism would in general inherit the pecu- 

 liarities, and repeat the life-history of its grandparent- 

 ceU, 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 fortimate, occurred, whereby the four 

 grand-daughter cells remained adherent until repro- 

 duction ; 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 repro- 

 duction (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 multicel- 

 lular 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 func- 

 tions 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 com- 

 bined 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 



