THE ORIGIN OF THE ANIMAL KINGDOM. 6$ 



reached the single cell. There is, perhaps, as broad a 

 series of phenomena to be studied between the single 

 cell and the real unit of life, as those which we have 

 found between the higher animals composed of many 

 cells and the single cell. Animals have all been 

 reduced to complexes of cells, but the cell bids fair 

 to be still farther reduced to a complex of inconceiv- 

 ably small granules. Be that as it may, it still remains 

 a fact that in the history of life we have not yet 

 discovered any direct evidence of an earlier condition 

 than that of the unicellular organisms. Embryology, 

 which is our first source of evidence, gives us a 

 record of a unicellular condition, but of nothing 

 earlier than this. The first step that we can take in 

 narrating the history of life is, therefore, to establish 

 the period when unicellular animals were the highest 

 condition of organic life in existence. The diffused 

 protoplasmic mother of life, if ever such existed, had 

 become broken into independent masses. 



Of course, nothing can be determined in regard to 

 the variety of form of these early organisms, nor can 

 we say whether they were all alike or whether they 

 showed variety like that of the unicellular organisms 

 to-day. Of their habits we know nothing. That 

 each organism was independent and capable of car- 

 rying on all the functions of life within itself follows 

 from every source of belief. That these organisms 

 multiplied by simple division is certain from the fol- 

 lowing reasons : Multiplication by division is a uni- 

 versal phenomenon in all living things to-day. In its 

 simplest form it is shown by most of the existing 

 unicellular animals, as is. illustrated in Fig. 4. As 

 5 



