180 THE OCEAN 



climate over the whole ocean, we cannot but 

 suppose that the deep sea would be unfavour- 

 able for animal life owing to the want of 

 circulation and atmospheric oxygen, but it is 

 probable that the same or nearly allied 

 species of benthonic animals were almost 

 everywhere present in the shallow-water 

 zones. When cooling at the poles set in, 

 those animals with pelagic larvae would be 

 killed out or be forced to migrate towards 

 the warmer tropics. By being able to limit 

 the reproductive process to the summer season, 

 some of these organisms with free- swimming 

 larvae have been able to live on in the tem- 

 perate regions, but in the tropical and coral- 

 reef regions we have the remnants of a once 

 universally-distributed shallow-water fauna. 

 With the disappearance of this shallow- 

 water fauna from the polar regions its place 

 would be occupied by the organisms from the 

 deeper mud-line, very few of which have 

 pelagic larvae. In this way we may account 

 for the similarity between the polar marine 

 faunas and floras, the great abundance of 

 individuals and the relatively few species 

 in the polar areas when compared with the 

 tropical area, as well as the greater likeness 

 of the shallow- water polar animals to deep-sea 

 species. From another point of view we 

 might suppose rather cold water to be uni- 



