ZOOLOGICAL REALMS 477 



the Hawaiian Group in this regard bears an unmistakable 

 resemblance to Japan. 



In all of these instances, and in others that might be 

 mentioned, the evidence unmistakably leads to the con- 

 clusion that while some immigrants from the mainland 

 may reach a new territory and perish owing to unfavor- 

 able conditions, others, being better fitted, have persisted. 

 The new surroundings — soil, climate, vegetation, and 

 enemies — are different from those of the home land, 

 however, and in these isolated districts the individuals 

 of the species gradually undergo modifications as the 

 centuries pass until they differ markedly from the 

 original ancestors. 



Zoological Realms. — The continents and islands of 

 the earth, from the frozen north and south to the 

 burning tropics, are to-day inhabited by more than a 

 million different species of animals, which in past times 

 have crossed barriers and become adapted to their sur- 

 roundings. And in much the same way that the earth 

 is divided into large physically different regions, so the 

 animals have roughly been included in five great realms 

 on the basis of their distribution. These are the Hol- 

 arctic. Neotropical or South American, Ethiopian, Ori- 

 ental, and Australian realms. The conditions peculiar to 

 each of these large divisions serve as a barrier to the 

 greater number of species of an adjacent realm, although 

 there are a few (except in the case of the Australian 

 realm) that intermingle along the border line and render 

 the boundary somewhat vague and indistinct. 



HoLARCTic Realm. — The Holarctic realm comprises 

 approximately all of the land surface north of the Tropic 

 of Cancer, and accordingly includes nearly the whole of 

 North America, all of Europe, and Asia north of the 

 Himalayas. No other region of the globe is so highly 

 diversified with tracts of perpetual ice, immense treeless 

 districts, deserts, swamps, and an almost endless variety 

 of soil conditions. 



