254 DOCTRINE OF EVOLUTION 



in the sense that Amoeba is, for they only have a place 

 in the chain of consecutive generations of Volvox 

 colonies. From the standpoint of the nutritive indi- 

 vidual it is better to be relieved of the reproductive task 

 in order that there may be no interruption of its special- 

 ized activities for the good of all, but the entailed mor- 

 tality is certainly disadvantageous to it. It is the higher 

 interest of the colony as a whole that supersedes the 

 welfare of the parts taken singly, and this larger welfare 

 is safeguarded by a differentiation worked out by natu- 

 ral evolution which results in the assignment of personal 

 and racial duties to different individuals, at the cost 

 ultimately of the lives of the former. 



We now reach the realm of the true many-celled 

 animals, or Metazoa, where the biological units are 

 combined to form an organic association displaying many 

 more resemblances to a human society. The fresh- 

 water polyp Hydra, like the foregoing illustrations, is 

 one whose structure has already been discussed in the 

 earlier chapters, but now we may use it for an analysis 

 of another series of biological phenomena. Its sac-like 

 body consists of two cell-layers ; the outer one is con- 

 cerned primarily with offense and defense, while the 

 inner layer is made up of digesting or nutritive elements. 

 The essential cells concerned solely with reproduction 

 lie below the outer sheet. Comparing this animal with 

 an association like Volvox, we discover the same differ- 

 entiation into immortal germ-elements and mortal 

 cells, concerned respectively with the Hydra^s racial 

 existence and with its individual life ; but far-reaching 



