SOCIAL EVOLUTION AS A BIOLOGICAL PROCESS 255 



changes have come about in the biological relationships 

 of the second class of cells. In describing the new phe- 

 nomena it is absolutely necessary to employ the terms 

 of human social organization, because the Hydra s 

 body is a true colony of diverse cells in exactly the same 

 sense that a nation is a body of human beings with more 

 or less dissiinilar social functions. 



To begin with the differentiation into ectoderm and 

 endoderm, the organism is comparable to a human com- 

 munity made up of military and agricultural classes. 

 The cells of the former group protect themselves and 

 the feeding elements also, w^hile the units of the second 

 defenseless type devote themselves to the task of pro- 

 visioning the w^hole community, giving supplies of food 

 to the defenders in exchange for the protection they 

 afford ; each kind needs the other, and each performs 

 some distinctive task for the other as well as for itself. 

 But the parallel thus drawm need not stop here. In the 

 case of the outer layer, the cells are mostly flat covering 

 elements that are the first to be torn off and injured 

 when the animal is attacked. Scattered about among 

 them are sense-cells standing like sentinels with deli- 

 cate upright processes which receive stimuli from with- 

 out ; the sense-cells transmit impulses to the network of 

 nerve-cells below, which is a counterpart of the signal 

 corps of an army, keeping all parts of the whole organi- 

 zation in communication with one another. Most 

 wonderful of all are the stinging-cells of the outer layer ; 

 these produce a flask-shaped, poisoned bomb which is 

 discharged by the convulsive contraction of the cell it- 

 self so as to stun and injure the enemy or prey. The 

 bomb-throwing cells die immediately after they have 



