It is impossible for the candid mind to dwell for a moment 

 on the fact that Britain produces only about one-fortieth part 

 of the ferns already known as inhabiting the globe, without 

 perceiving the impracticability of arranging that fractional 

 part in anything like a connected series. Select one British 

 species, CapUlus -Veneris, for instance, and we shall find that 

 there are at least a hundred exotic species which approach it 

 more closely than any that occur in Britain : therefore, assum- 

 ing that two thousand ascertained ferns constitute a connected 

 chain, it follows that in Britain a hundred links are wanting 

 at that part of the chain where Capillus-Veneris is situate. 

 There is still another mode of accounting for some of the 

 monstrous gaps observable in the chain of species. The 

 physical changes perpetually occurring in the condition of the 

 earth's surface, render large tracts of land incapable of sus- 

 taining any longer certain species which formerly hid the soil 

 with their luxuriant foliage : we know that thousands of such 

 species did exist, and do not exist ; but that their history is 

 preserved for ever in Geology, that glorious book whose pages 

 are traced by Nature's own hand upon tablets of adamant. 

 The hypothesis that Nature is compensating her losses by new 



