n] BIOLOGICAL FOUNDATIONS 47 



end of the front half. As in the growth of the 

 hydroid colony, the old organization is kept entire 

 and whole, the new organization is built up in the 

 bud; here, however, it is not one whole individual 

 giving rise to another, but one half giving rise to just 

 that dissimilar half which is its complement. What 

 is more, both the unlike halves of the original whole 

 can thus add what is wanting that and no more 

 to bring them up to the rank of wholes again. There 

 is an old school-boy question about a cricket bat : 

 suppose the handle of a bat broke, and a new one 

 was put on .to the old blade. Suppose then that the 

 blade broke and was in its turn replaced ; would the 

 bat still be the same bat ? That is a hard question, 

 but Clavellina asks a harder still. 



From this to the extremes of regeneration, such 

 as occur among flatworms and protozoa, is another 

 large step. Stentor, for instance (a protozoan which 

 happens to be specially convenient for experimental 

 purposes), may be chopped, broken, or shaken up 

 into pieces of all sizes and shapes, and every piece, 

 provided only that it is above a definite minimum 

 size (less than ^ inch in diameter, and in bulk only 

 1 or 2 per cent, of a full-grown Stentor), and that it 

 contains a piece of the nucleus, will blossom out as 

 a minute but fully-formed individual, which will feed 

 and grow and be indistinguishable from a product of 

 natural generation. 



