INTRODUCTION. 



In looking from any stand-point, at the variety of 

 organic forms, the theory of evolution will be the 

 most successful in explaining the resemblances and 

 differences of organized life. 



Our theories about the creative power or the first 

 impulse, may differ in all other points, but in one 

 all scientists have instinctively agreed, and that is 

 the method of explaining variations by comparing 

 one form of organic life with another, and deriving 

 one from, or out of the other. In the same meas- 

 ure in which our knowledge of living organisms 

 increased and the remains of extinct ones were dis- 

 covered, the idea of evolutionary progress took 

 more and more hold of the scientific mind and be- 

 came clearer and better defined. 



Evolution from a given point may take place in 

 different lines, and has done so from the very be- 

 ginning ; the first divergence being the formation 

 of the vegetable and animal kingdoms. We can 

 easily account for this first and fundamental split, 

 by the fact that the animal absorbs exactly the sub- 

 stance secreted by living vegetation, and vice versa. 

 Any aquarium will furnish the proof of this, as the 

 water must be constantly renewed if either animals 

 or plants are kept exclusively, but if tbey are kept 



