GEOGRAPHICAL DISTRIBUTION OF ORGANISMS. 477 



arctic zones north are the same as those found in similar 

 zones south. It is, again, as if each group was created 

 as it is and where we find it, and prevented from pass- 

 ing to similar zone, north or south, by the torrid tem- 

 perature intervening. 



CONTINENTAL FAUNAL REGIONS. 



Thus far we have considered only temperature con- 

 ditions; now we take up other limiting conditions. 



If continuous 

 land existed all 

 around the earth, 

 then, barring des- 

 ert regions, there 

 is no reason to 

 doubt that species 

 would range all 

 around, and there 

 would be a strict- 

 ly zonal arrange- 

 ment of species de- 

 termined by tem- 

 perature alone. 

 But the continents 

 are widely and im- 

 passably separat- 

 ed by oceans. 

 Therefore the species on different continents are wholly 

 different. It is, again, as if each continent had been 

 populated by its own inhabitants, suited to its climate, 

 just as we find it now, and had not been able to cross 

 the ocean barrier and mingle. 



If we take the facts in detail the case becomes still 

 stronger. Let Fig. 334 represent a north polar projec- 

 tion of the earth. The five zones are represented by 



Fig. 334. — Polar projection of the earth : 1, 

 tropical ; 2, temperate ; 3, subarctic ; 4, arc- 

 tic ; s, polar regions. 



