THE GEOLOGICAL RECORD. 95 



gence or origin of a group may be but vaguely inferred, 

 so we have to compromise and adopt divisions which only 

 indicate progressive grades of structure. 



There is one more consideration to be borne in mind 

 when dealing with species. The living species around us 

 represent the extreme tips of the branches of the phylo- 

 genetic tree, which have succeeded in the struggle for 

 existence. If their ancestors are extinct these living 

 twigs become isolated from each other, and so real limits 

 become established, which of course would disappear if 

 the complete series of extinct forms were discovered. It 

 is the inevitable result of evolution by the elimination 

 of the less well-adapted that ever widening gaps come 

 to separate the diverging Ijving representatives of the 

 various branches derived from a common stem. 



CHAPTER VIII 



THE GEOLOGICAL RECORD OF SUCCESS AND FAILURE 



EVOLUTION is too often represented as a history of success 

 and progress ; it is also one of extinction and failure. 

 Authors, seeming to forget that for one line of develop- 

 ment that succeeds there are a hundred that fail, are fond 

 of invoking some mysterious guiding principle, some in- 

 ternal perfecting agency an 'dan vital, or what not to 

 account for evolution ; but there would appear to be 

 little scope for such mysterious forces in a world where 

 the majority of individuals are crushed out, where most 

 lines of development fail hopelessly to establish them- 

 selves. What guiding principle there may be behind the 

 whole of creation is a subject outside the scope of Natural 

 Science, and on which it can express no opinion. It 

 cannot even prophesy whether man or the bacillus will 

 eventually triumph in the struggle for existence ; indeed, 



