442 GENERAL BIOLOGY 



two sorts first named. The feeble beginnings of this 

 differentiation we have noted in the hydra (yage i6i), and 

 further development has been briefly traced in the earth- 

 worm (especially in the nerve cells, figure 109), and in the 

 salamander. 



The cells of a metazoan make common cause of their rela- 

 tion to the outside world, and, with more materials out of 

 which to build, greater progress in differentiation is made. 

 Systems of organs arise; digestive, circulatory, respiratory 

 and excretory systems for performing the nutritive pro- 

 cesses of the body, and nervous and muscular systems, for 

 the control and coordination of all, and for maintaining 

 proper relations with the outside world. 



Intercommunication without nerves. — Our studies shall 

 be of this specialized apparatus; but we will do well to 

 remember that receptivity and action are older than nerve 

 and muscle. These functions are too general and too 

 important to be wholly committed, even in the highest 

 animals, to any particular set of cells. It is the more 

 specialized activities that are taken over by these cells, but 

 there remain other important functions of relation that are 

 yet fulfilled by slower processes in which nerve and muscle 

 take small part. Manifestly, before nerve and muscle were 

 developed interaction had to be brought about by contact 

 of adjacent cells, as is still necessary during early embryonic 

 development. The movement of a stimulus passing from 

 cell to cell was like that of particle upon particle in the body 

 of a pricked amoeba (fig. 256) . It is like the slow communi- 

 cation by word of mouth from door-yard to door-yard, as 

 compared with the telegraphic communication of news. 

 When nerves develop they take on wholly the function 

 of rapid communication between distant parts ; but certain 

 of the other older and slower processes of the bodily economy 

 continue to be performed by older and slower methods. 



