70 THE LIVING WORLD. 



forming only a part of the functions of life, and thus 

 each dependent upon the others. The whole forms 

 a unit. In such a community it is no longer possible 

 for a single cell to be separated from the rest and 

 still continue its life, for such a cell would be able to 

 perform only those duties for which it was adapted, 

 and it would therefore soon die. 



The Gastrcza, the Common Trunk of the Animal 

 Kingdom. 



That such a multicellular community arose in the 

 early history of life from the unicellular forms, is of 

 course evident, and that it arose by cell division 

 which did not become complete enough to separate 

 the individual cells from each other, is also almost 

 certain. Exactly how a division of labor first arose, 

 or what that first division may have been, it is perhaps 

 impossible to say. Indeed, it is not improbable that 

 in the early history of life there may have arisen 

 many different types of division of labor giving rise 

 to different kinds of true multicellular communities. 

 But whatever may have been the early varieties of 

 such differentiation, the very important fact is true 

 that only one of these early types of multicellular 

 animals perpetuated itself so as to affect the subse- 

 quent development of animals. The study of embry- 

 ology shows that all of the subsequent types of the 

 animal kingdom arose from one definite type of early 

 differentiation. In other words, of all the types of 

 cell aggregates or communities which may have been 

 produced in the early history of life only one of them 

 proved itself of value enough to take a definite place 



