130 DYNAMICAL GEOLOGY. 



and character of bottom. The most remarkable of these 

 are those inhabiting deep-sea bottom, and forming an 

 abyssal fauna. Again, about the shores of isolated islands, 

 as Madagascar and Australia, the marine fauna are as 

 peculiar as the land fauna. 



Origin of Geographical Diversity. 



Until recently the most reasonable view seemed to be 

 that species originated where we find them, and spread in 

 all directions as far as they could. According to this 

 view, the diiference between faunas ought to be strictly 

 in proportion to the impassableness of the barriers be- 

 tween. This is largely true, but does not account for all 

 the phenomena. There is another element of equal im- 

 portance, viz., the time during which the harrier has 

 existed. It is probable that faunas and floras are subject 

 to slow, progressive changes, taking different directions 

 in different places. If there be no barriers, spreading by 

 dispersal or migration prevents extreme diversity. But 

 if a barrier be at any time set up by geological changes, 

 then diversity commences, and increases with time. 

 According to this view, the Australian fauna is so peculiar 

 because this continent has been so long isolated from all 

 others. The fauna of islands off the coasts of continents 

 are often very similar to that of the adjacent mainland, 

 because they have only recently been separated. Thus, 

 for example, the fauna and flora of the British Isles 

 differ but very slightly from those of the Continent, 

 because, as we now know, these islands, even since their 

 inhabitation by man, have been in full connection with 

 Europe. The divergence has commenced, but is only 

 varietal, not specific. This subject will be taken up 

 again, and more fully explained in connection with glacial 

 epoch, p. 403. 



