CHAPTER II 



UNICELLULAR AND MULTICELLULAR ANIMALS 



WE must now turn to the main project of this book, which is 

 to attempt to trace out the lines along which animal Evolution 

 has proceeded, with special reference to that particular line 

 which leads up to man. Indeed, we shall have to stick somewhat 

 closely to this one main highway, and can but barely pause to 

 glance along the numerous branch roads, interesting though the 

 travelling there might be. 



It is perhaps necessary to say, at the outset, that the history 

 of the Evolution of man cannot be written as a plain, matter-of- 

 fact tale. Many portions of this history are tolerably well under- 

 stood, but there are other periods, in some of which notable steps 

 of progress were made, of which no record has ever been discovered. 

 We must therefore expect occasionally to be reduced to specula- 

 tion, and here and there to meet with controversy and with 

 opposing theories. 



It is not proposed here to enter into any full discussion as to 

 the origin of life. It may shortly be said that in the existing 

 state of knowledge, no very definite theory is possible. We 

 know that life is associated with a jelly-like or semi-fluid substance 

 called protoplasm, which consists of a very complex mixture of 

 albuminoids. These albuminoids are continually undergoing 

 changes and interactions of a complex kind, the sum total of which 

 constitutes life. Many of these reactions have been reproduced, 

 or imitated, artificially, and have been shown to be purely chemical 

 or physical. The chemical nature of the albuminoids is indeed 

 so complex that some considerable time must yet elapse before 

 it can be completely investigated ; and until such time it is obvious 



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