GEOGRAPHIC DISTRIBUTION OF ANIMALS: CONTINENTS 277 



for our present purposes. They demonstrate a pattern of variability ex- 

 hibited by species. This variabiUty throughout a geographic region is most 

 eloquent of evolution, as Darwin recognized. The phenomenon is exactly 

 what we should expect if dispersal from a center, accompanied by evolu- 

 tionary change, is a fact. If, on the other hand, all these neighboring, 

 slightly differing, forms were separately created, what a vast multitude of 

 separate creations must have occurred! 



The variability of species just mentioned forms the basis upon which spe- 

 cies are commonly divided into geographic races or subspecies (see pp. 

 320-327 for additional discussion). 



Conclusions 



We have seen that similar continents are not necessarily populated with 

 similar animals. Similarity among continents in animal inhabitants de- 

 pends far more upon the accessibility of those continents to the same ani- 

 mals than it does upon mere similarity of living conditions. Accordingly, 

 the faunas of northern continents are more similar to each other than the 

 faunas of southern continents are similar to each other or to the faunas of 

 the northern continents. This relationship is explained by the relative ac- 

 cessibility of all parts of the Northern Hemisphere, contrasted with the 

 relative inaccessibility of the main land masses in the Southern Hemi- 

 sphere. 



The role played by accessibility is entirely understandable if animals 

 are found where they are, not because they were created on the spot, but 

 because they could get there. Presumably some animals originate in a 

 certain region and never disperse far from their point of origin. But such 

 a stay-at-home history is the exception, not the rule. Most animals have 

 attempted to expand their ranges, the attempts meeting with greater or less 

 success depending upon the barriers encountered and upon the versatility 

 and adaptability of the organisms themselves. 



As animals move into new regions success is frequently achieved by 

 adoption of bodily changes in structure and function. The changes may be 

 great enough so that the resulting forms can no longer be considered to 

 belong to the same species as the stock from which they arose. Such new 

 races or species will, however, retain many points of similarity to each 

 other and to the parent form. Since, at first at least, these kindred species 

 occupy adjoining territories, the phenomenon of similar species replacing 

 each other as one travels across the face of a continent arises. Eventually 

 the original and derived species may become extinct throughout a consid- 



