THE HIGHER ORGANISMS 123 



that in most cases their homologues can be found in 

 exceedingly elementary forms of life. 



The Reproductive System. It has already been sug- 

 gested that reproduction is at the very foundation of 

 cell association, and therefore is one of the first, if not 

 the first, specialization of the composite organism. At 

 first it is so indefinite that any cell may apparently take 

 on the function when the .appropriate moment is at 

 hand and the germinal cells seem to be widely scattered 

 among the somatic cells, but gradually they become 

 collected into groups gonads which form the founda- 

 tion of the reproductive organs. A future chapter will 

 be devoted to the subject of reproduction. 



The Digestive System. Unicellular organisms nourish 

 themselves as they perform all other functions, through 

 activities of their own. Nutritious materials absorbed 

 or ingested into their substance meet solution by diges- 

 tion and subsequent assimilation in the cell, the residuum 

 being extruded. Among the colonial protozoa the cells 

 may or may not retain this independence. Thus in epis- 

 tylus and carchesium, we have no reason to suppose 

 that any individual contributes to the support of any 

 other. In microgromia, however, it may be that cells that 

 are more successful in gathering up nutritious matter 

 give up some of the proceeds to the less successful cells 

 through their protoplasmic connections. Among vege- 

 table cells community of interest in regard to nutrition 

 appears very early and assumes great importance for 

 what is seen among the most lowly forms of animal life, 

 i.e., the transmission of nutritious pabulum from cell to 

 cell persists even among the highest forms of vegetable 

 life. 



Among the elementary animal composits the differ- 

 entiation of function is not sufficient to prevent almost 

 any cell yielding to primitive tendencies. Thus among 

 the porifera the digestive cells constantly, and in hydra 

 not infrequently, take useful particles directly into their 

 own substance. This is particularly interesting in 



