302 ON THE ORIGIN OF SPECIES 



has been much discussion whether recent forms are 

 more highly developed than ancient. I will not here 

 enter on this subject, for naturalists have not as yet 

 defined to each other's satisfaction what is meant by 

 high and low forms. The best definition probably is, 

 that the higher forms have their organs more distinctly 

 specialised for different functions ; and as such division 

 of physiological labour seems to be an advantage to 

 each being, natural selection will constantly tend in so 

 far to make the later and more modified forms higher 

 than their early progenitors, or than the slightly 

 modified descendants of such progenitors. In a more 

 general sense the more recent forms must, on my 

 theory, be higher than the more ancient ; for each 

 new species is formed by having had some advantage 

 in the struggle for life over other and preceding forms. 

 If under a nearly similar climate, the eocene inhabit- 

 ants of one quarter of the world were put into com- 

 petition with the existing inhabitants of the same or 

 some other quarter, the eocene fauna or flora would 

 certainly be beaten and exterminated ; as would a 

 secondary fauna by an eocene, and a palaeozoic fauna 

 by a secondary fauna. I do not doubt that this 

 process of improvement has affected in a marked and 

 sensible manner the organisation of the more recent 

 and victorious forms of life, in comparison with the 

 ancient and beaten forms ; but I can see no way of 

 testing this sort of progress. Crustaceans, for in- 

 stance, not the highest in their own class, may have 

 beaten the highest molluscs. From the extraordinary 

 manner in which European productions have recently 

 spread over New Zealand, and have seized on places 

 which must have been previously occupied, we may 

 believe, if all the animals and plants of Great Britain 

 were set free in New Zealand, that in the course of 

 time a multitude of British forms would become 

 thoroughly naturalised there, and would exterminate 

 many of the natives. On the other hand, from what 

 we see now occurring in New Zealand, and from 

 hardly a single inhabitant of the southern hemisphere 



