THE ORIGIN OF THE ANIMAL KINGDOM. 8$ 



field lying below the simple cell : but none of our 

 sources of evidence give us as yet any sure traces 

 of this earlier history. Of the unicellular stage of the 

 history we are sure. Without being able to deter- 

 mine much in regard to them, we may perhaps look 

 upon the unicellular organisms existing to-day as a 

 tolerably correct picture of those of early times. 



Second : We learn to picture to ourselves these 

 early unicellular forms as multiplying by division, 

 and we see among them many instances where the 

 multiplication did not produce complete separation 

 of the cells. The cells remained attached to each 

 other and formed colonies of unicellular animals. 

 Among these colonies a differentiation of the cells 

 and of function began to make its appearance. To 

 what extent different types of this early differentia- 

 tion appeared, we do not know, but finally there 

 arose one which has proved permanent. This was 

 the two-layered cup to which has been given the 

 name of Gastraea. This simple type with its mouth 

 and stomach, proved itself strong enough to battle 

 for life with nature, and it probably became the 

 starting-point of all subsequent animals. The 

 kingdom of plants separated itself permanently 

 from the animals, and can no longer be traced with 

 them. 



Third ; The Gastraea was the last point of com- 

 mon origin to which the different sub-kingdoms can 

 be traced. From here there arose a divergence 

 which soon produced the types of the existing 

 animal world. Moreover, we- have seen that not 

 only is it impossible to trace the whole animal world 



