sTKATKiKAI'lllrAI. I'AL^ONTOLOGY 15 



clear line of demarcation has aa yet been drawn between special 

 and sub-species that is, the forms which in the opinion of some 

 naturalists come very near to, but do not quite arrive at, tin- 

 rank of species ; or again between sub-species and well-marked 

 varieties, or between lesser varieties and individual diilV-rencea ; 

 and a series [of specimens] impresses the mind with tin- idea of 

 an actual passage." Further on he says, "From these remarks 

 it will be seen that I look at the term species as one arbitrarily 

 given, for the sake of convenience, to a set of individuals closely 

 resembling each other, and that it does not essentially differ from 

 the term variety, which is given to less distinct and more fluctuat- 

 ing forme." 



Darwin and Wallace developed their views about the same 

 time, and both accounted for the existence of so many varieties, 

 species, and genera of organisms by indicating the various causes 

 and conditions which influence all kinds of organisms and tend to 

 modify their habits, forms, and members. They have shown how 

 the constant struggle for existence and the frequent changes of 

 environment may have developed or have increased a tendency 

 to variation, and how this variation results in the formation of 

 varieties and species. Natural selection (or the selective action 

 of natural processes) results also in the extinction of many forms 

 and in the survival of a few namely, of those few which an- tin- 

 most capable of accommodating themselves to the more rigorous 

 and changed conditions of existence. This is called the " survival 

 of the fittest." It is by the infinite repetition of these processes 

 that species which are now widely different may have been evolved 

 from a common ancestor. 



Another consequence of natural selection is the preservation of 

 those varieties and species which chance to possess ptviiliaritit-s 

 that are useful to them in the struggle for existence. Thus as 

 Darwin says (op. cit. p. 103), "it leads to the improvement of each 

 creature in relation to its organic and inorganic conditions of life, 

 and consequently in most cases to what must be regarded as an 

 advance in organisation." In other words, it leads to the evolution 

 of higher and higher forms of life. 



Now if the descendants of varieties may become different 

 species, and if the descendants of different species may in course 

 of time become so differentiated that most naturalists would rank 

 them as different genera, and if this process has been going on 

 during the whole of geological time, since the world first became 

 fit to support life; then we might expect that the successive 

 assemblages of fossils preserved for us in the rocks of the earth's 

 crust would furnish us with a long succession of links between one 



