CHAPTER XVI 



The continuity of life The conception of species The principles of 

 taxonomy The taxonomic evidence of organic evolution. 



IN accordance with the theory of evolution we may picture 

 to ourselves the entire animal and vegetable population of the 

 earth, both past and present, as forming one vast, tree-like 

 organism, all parts of which, if we knew enough about their 

 history, could be traced into actual, although of course not simul- 

 taneous, protoplasmic connection with all other parts. This 

 tree commenced its growth far back in geological time, and its 

 branches became ever more and more ramified in succeeding 

 ages, and more and more diversified in character as they 

 diverged from one another. Only the youngest twigs of the 

 tree, however, are still actually alive, being represented by the 

 animal and vegetable population of the earth for the time being. 

 All the older parts have died away, most of them without leaving 

 any traces of their former existence, and thus has arisen the 

 actual discontinuity which is found to occur between the sur- 

 viving groups of organisms at any given period. 



The tendency to structural variation which all organisms 

 exhibit, whatever may be its cause, is responsible for the 

 progressive diversity which is gradually set up between the 

 different branches of the organic tree. The combined action of 

 the forces of heredity and variation bring about " descent with 

 modification." How it is that such modification always leads, 

 in the long run, to a more or less perfect and often very 

 specialized adaptation of both plants and animals to the con- 

 ditions under which they live will be discussed subsequently. 

 At the moment we have to deal with things as we actually find 

 them. 



In any scheme of zoological or botanical classification the 

 lowest unit must, of course, be the individual. The untrained 

 observer, acquainted only with the more familiar plants and 

 animals, sees no difficulty in arranging these in apparently 



