LIFE ON THE EARTH. 199 



This process, in the careful hands of Mr Davidson, 

 has already reduced the number of species in several 

 groups of Brachiopoda; and augmented the geogra- 

 phical area of the groups. When fully carried out, I 

 expect it will be found that the Brachiopoda of the 

 Carboniferous limestones, so remarkably allied over 

 all Europe, have a much larger agreement with those 

 of North America than is at present allowed in our 

 Catalogues. 



In this process of reducing the admitted number 

 of species we are following in the track of great 

 botanists like Bentham and Hooker, whose researches 

 in modern nature have brought them into view of 

 similar difficulties. 



But what if, as naturalists of no mean fame have 

 told us, the limits we thus draw round species are 

 arbitrary functions of our own minds, not real 

 boundaries set by nature; limits which are not 

 chosen alike by different minds, nor surely and firmly 

 retained by the same mind, at different times? If 

 such an opinion, whether true or not, were accepted 

 by physiologists from a survey of existing nature, 

 how would it affect our view of the succession of 

 life on the globe? Those who maintain it to be 

 true, usually assume some hypothesis which involves 

 the operation of long time, and thus brings the 

 question within the judgment of Geology. 



