PROCESSES OF EVOLUTION 



249 



there is no ground for supposing that this limitation in habitat is, in 

 general, caused by an adaptation to a very narrow range of conditions 

 which are realized in that spot and nowhere else, since if that were the 

 case we should expect to find that if the territories of two rare species 

 overlapped at all they would be exactly the same, which is not true. 

 Further, it is difficult to believe that these rare species all represent the 

 last remnants of species which were formerly much commoner and 

 more widespread, since then there should be a fair number whose 

 range consists of several isolated patches, which again is not true. 



These considerations led Willis to suggest that the rare species 

 occurring in a single isolated area had in fact been newly formed. He 



Fig. 110. Age and Area. — Maps 

 of Ceylon showing the areas of 

 distribution of very rare (VR), 

 rather rare (RR), and rare (R) 

 endemic plant species. Where a 

 species occurs in two separate 

 localities, they are connected by 

 a wavy line. 



(After Willis.) 



put forward the following hypothesis, known as the theory of "age and 



area 



"The area occupied at any given time, in any given country, by a 

 group of allied species at least ten in number, depends chiefly, so 

 long as conditions remain reasonably constant, upon the ages of the 

 species of that group in that country, but may be enormously modi- 

 fied by the presence of barriers such as seas, rivers, mountains, 

 changes of climate from one region to the next, or other ecological 

 boundaries, and the Hke, also by the action of man, and by other 



causes. 



Roughly, then, in a statistical way, the older the species, the greater the 

 area it occupies. 



The importance of this extremely simple, but within its limits 

 plausible, hypothesis for our present discussion is this. The narrowly 

 restricted range of some of the rare species which the age and area 

 hypothesis identifies as new species indicates that these species have 

 had a sudden and local origin in the recent past. The evidence strongly 



