THE ORIGIN OF THE ANIMAL KINGDOM. 67 



become lost in thousands of small twigs. From the 

 time of branching it is of course impossible to follow 

 the history as a whole, and only possible to follow 

 that of the various branches. At the point which 

 we have now reached we have come to the end of 

 the trunk, and we find the first division into two large 

 branches, animals and plants, as mentioned in the 

 preceding chapter. The separation between these 

 two kingdoms seems to have occurred even while 

 the living world consisted of unicellular organisms. 

 From this point we must follow the animals and 

 plants separately. We shall first take up the his- 

 tory of animals, leaving that of plants for later 

 consideration. 



Early History of the Animal Kingdom. The Origin 

 of Multicellular Animals. 



The Protozoa as they exist to-day may be fairly 

 supposed to illustrate the early unicellular animals 

 of pre-historic times. The next stage in the history 

 of life was the appearance of the multicellular organ- 

 isms. It is again the study of the unicellular ani- 

 mals to-day which gives us suggestion as to how 

 this change arose. Among most of the unicellular 

 organisms, when a cell divides, the parts completely 

 separate from each other and live subsequently inde- 

 pendent lives. Among a few living forms, however, 

 the parts do not completely separate from each other, 

 but remain together in some sort of connection and 

 are more or less dependent upon each other. Fig. 5 

 shows such a group of cells, the members of which 

 have arisen from the division of a single one, and which 



