226 AFFINITIES AND GEOGEAPHICAL 



organisms spread over different regions have been observed. 

 It is observed, for instance, that there is more uniformity be- 

 tween two masses of land which are both in the north or south 

 hemisphere, albeit divided by wide oceans, than between 

 two portions of one continent extending into both of these 

 hemispheres. North America is zoologically less allied to 

 South America, than it is to Northern Europe. An island, 

 however far apart, usually shows zoological features reflective 

 of those of the nearest continent. Two countries, again, 

 divided only by a narrow sea, have generally the same 

 flora. 



Some principle affecting the development of the higher 

 animals can also be detected in connexion with geological 

 chronology. Startling as it may appear, we are now assured 

 that the present great continent comprising Europe, Asia, and 

 Africa, has been, with minor changes in the relative position 

 of sea and land, one theatre of organic being since the com- 

 mencement of the existence of land animals upon the surface 

 of the earth ; that is to say, there has been, on one part or 

 another of this geographical area, an uninterrupted chain of 

 living forms from an early period in the secondary formation. 

 This is the zoological province whose history is presented by 

 the geologist ; it is the oldest we are acquainted with. There 

 are, however, some isolated regions which are known with cer- 

 tainty to have been in a condition of dry land for a less space 

 of time. Such are the volcanic islands, of which the Isle of 

 Bourbon is an example. Such also are the Galapagos Islands, 

 placed in the Pacific, above five hundred miles from South 

 America. Now it is remarkable in such regions to find the 

 mammalia either wholly wanting or in very small numbers. 



Australia itself a fifth great section of the habitable globe 

 appears to be one of these regions of an incomplete zoology. 

 It is well known to have no native mammalia besides that 

 humble implacental kind which are nearly peculiar to it, and a 

 few rodents and bats. Professor Owen remarks how the 

 fishes of the oolitic era acrodus, psammodus, etc. with the 

 contemporary mollusks (trigonise and terebratulre), which 

 served these fishes for food, are represented in the living 

 cestracion which swims the Australian seas, with exactly the 

 same sea mollusks to yield them sustenance. " Araucariae and 

 cycadeous plants likewise," he says, " flourish on the Australian 

 continent, where marsupial quadrupeds abound, and thus ap- 

 pear to complete a picture of an ancient condition of the 



