I 



XXV.] GBNEKAL EEMAKKS ON DEVELOPMENT OF SPECIES. 333 



believe the geological evidence is too scanty and frag- 

 mentary to be of miicli value ; ^ but what we have is cer- Geological 

 tainly favourable to the belief that there has been, on the flours the 

 whole, an advance in grade of organization during geolo- belief in 

 gical time. Thus, reptiles have been superseded, to a great „ .. 

 extent, by warm-blooded animals ; the great marine rep- have given 

 tiles have given place to whales and other Cetaceans, the 3!^J° 

 great land reptiles to Mammalia, and the Pterodactyles, or Wooded 

 flying reptiles, to birds. This, however, says nothing either 

 for or against the theory of natural selection among spon- 

 taneous variations as the principal cause of improvement. 

 But I think that, on the whole, geological evidence is Geological 

 against that theory. Were it true — were natural selection argument 



" "^ against 



not only one cause of change, which it certainly is, but the Darwin's 

 principal cause — improvement ought to take place most ^'^^°^'^'- 

 rapidly in the classes where there is most variability, and 

 least fixity of form : in other words, selection ought to do 

 most where there are the most numerous and the greatest 

 variations to select from. Now, variability as between 

 individuals is greatest in the lowest classes : but this 

 does not cause the production of new species to go on 

 among them with any corresponding rapidity. Geologists, improve- 

 on the contrary, appear to be agreed that old species ™^^^ ^°^^ 

 disappear, and new ones come to take their places, most rapidly in 

 rapidly in the highest classes. I think this is an impor- glasses. '^^ 

 tant argument in favour of beKeving that advance in 

 development is due to some vital power which is most 

 energetic in the highest classes, and not to any mere 

 inorganic agency like natural selection. 



"We have to account not only for the fact of improve- 

 ment, or, in other words, for advance in grade of organiza- 

 tion, but for the fact that improvement goes on in divergent Diver- 

 lines. The organic tree not only grows higher, but branches ^^H ?ej. 

 out into classes. This, however, presents no difficulty, if 

 it is admitted, as the development theory requires, that if 



1 See the chapter, in Darwin's Origin of Species, on the Imperfection of 

 the Geological Kecord ; also " Illogical Geology," in the second volume 

 of Herbert Spencer's Essays. 



