ISOLATION 175 



Pteropoda, all floating forms with remarkably 

 simple surroundings, are, I believe, as constant in 

 their characters as other animals. But if examples 

 from the higher animals are thought to be necessary, 

 I would instance the Gannets, some of which are 

 white, while others are brown, yet both live together 

 in the same localities and have the same habits. It 

 is the same with the Terns, Albatrosses, and other 

 sea-birds. The colours here must be useless charac- 

 ters, and yet they are constant. The environment 

 of land animals and plants is so much more com- 

 plex than that of marine animals that we cannot feel 

 sure that those which live together have the same 

 habits ; but if Dr. Wallace's statement breaks- down 

 with the marine animals, it is sufficient. 



These so-called useless variations we suppose to 

 have been accumulated by definite variation, and 

 preserved by isolation, without any help from selec- 

 tion. 



Definite positions of organic stability. A ques- 

 tion which is being much discussed at the present 

 time is whether variations, after making a certain 

 amount of progress, slowly or rapidly, do not then 

 remain stationary. That is, is progress intermit- 

 tent ? 



It seems probable that many of the variations 

 which formerly took place are now stationary ; but 

 some are, certainly, still in progress. If the sta- 

 tionary characters are useful, it is easy to account 

 for the stability by natural selection, because, if 

 the external conditions remain constant, the useful 

 characters will not change. Natural selection is not 



