86 EVOLUTION 



THE BEGINNING OF A BODY. The simplest 

 organisms are single cells physiologically com- 

 plete in themselves; they leave off where 

 higher creatures begin, that is to say, in a 

 unicellular state; they do not form "bodies." 

 Here we have perhaps the greatest gap and 

 the greatest step in organic nature, that 

 between single-celled and many-celled organ- 

 isms. It is very interesting to inquire into 

 the beginning of a " body." What are the 

 possibilities ? 



We know of some simple units that have a 

 habit of coalescing into composite masses, of 

 others in which the nucleus divides over and 

 over again within the cell so that multi- 

 nucleate organisms are formed, and of others 

 again that break their definition, and do their 

 best to get beyond the unicellular state, by 

 forming loose colonies. It was probably in 

 the third of these ways that body-making 

 began. Certain simple organisms, unable 

 fully to complete that division into two or 

 more separate units which normally occurs 

 at the limit of growth, bridged what Agassiz 

 called " the greatest gulf in organic nature." 

 It was perhaps through some weakness that 

 the daughter-units, formed by division of 

 the mother-cell, remained associated, instead 

 of drifting apart in individual completeness. 



