Physical Geography. 491 



It is now generally admitted that we may safely reason 

 on such facts as these, which supply a link in the defective 

 geological record. The upward and downward movements 

 which any country has undergone, and the succession of such 

 movements, can be determined with much accuracy; but 

 geology alone can tell us nothing of lands which have entire- 

 ly disaj^peared beneath the ocean. Here physical geography 

 and the distribution of animals and jjlants are of the great- 

 est service. By ascertaining the depth of the seas separating 

 one country from another, we can form some judgment of 

 the changes which are taking place. If there ai-e other evi- 

 dences of subsidence, a shallow sea implies a former con- 

 nection of the adjacent lands ; but if this evidence is wanting, 

 or if there is reason to suspect a rising of the land, then the 

 shallow sea may be the result of that rising, and may indicate 

 that the two countries will be joined at some future time, 

 but not that they have previously been so. The nature of 

 the animals and plants inhabiting these countries will, how- 

 ever, almost always enable -us to determine this question, 

 Mr. Darwin has shown us how we may determine in almost 

 every case, whether an island has ever been connected with 

 a continent or larger land, by theipresence or absence of ter- 

 restrial Mammalia and reptiles. What he terms " oceanic isl- 

 ands " possess neither of these groups of animals, though they 

 may have a luxuriant vegetation, and a fair number of birds, 

 insects, and land-shells ; and we therefore conclude that they 

 have originated in mid-ocean, and have never been connected 

 with the nearest masses of land. St. Helena, Madeira, and 

 New Zealand are examples of oceanic islands. They possess 

 all other classes of life, because these have means of dispersion 

 over wide spaces of sea, which terrestrial mammals and birds 

 have not, as is fully explained in Sir Charles Lyell's " Prin- 

 ciples of Geology," and Mr. Darwin's " Origin of Species." 

 On the other hand, an island may never have been actually 

 connected with the adjacent continents or islands, and yet 

 may possess representatives of all classes of animals, because 

 many terrestrial mammals and some reptiles have the means 

 of passing over short distances of sea. But in these cases the 

 number of species that have thus migrated will be very small, 

 and there will be great deficiencies even in birds and flying 



