INTRODUCTION 9 



monest animals of to-day are those which have so far 

 proved victorious, and numerical strength is one mark 

 of dominance, a sure one, when it can be clearly 

 ascertained. 



Any group of animals or plants which is numeri- 

 cally very strong will be found to include a large 

 number of species, and these will be finely graded, 

 one species being with difficulty distinguished from 

 those which most nearly approach it. Dominant 

 species are usually combined into what we may call 

 continuous groups. It is only when a group is domi- 

 nant, or so to speak in easy circumstances, that many 

 nearly allied forms can maintain themselves side by 

 side within it. For declining groups there is a 

 rigorous selection. The principle applies to other 

 things beside natural species. It has become a proverb 

 that even bad workmen find employment when trade 

 is good. Growing industries, such as lithography or 

 paper-making, turn out an immense variety of forms 

 and qualities ; but articles which are going out of use, 

 say tallow candles or sand-glasses, can only be bought 

 in one or two patterns. Sealing-wax thirty years ago 

 was sold in an endless variety of colours, but the com- 

 petition of adhesive envelopes has now reduced the 

 number to one or two. I am told that while you can 

 get ample choice of cloth-covered buttons, you can 

 hardly buy more than one kind of polished steel 

 buttons, which have now gone out of fashion. 



All prosperous species, families, orders and classes 

 at least hold their ground. If they can be successfully 

 invaded, they are no longer dominant. Dominant 

 groups are therefore geograpJiically continuoits ; they 



