440 SCIENCE PROGRESS 



Interposition of barriers, such as mountains, broad rivers, 

 deserts, arms of the sea, sudden changes of climate 

 from one district to the next, and the like ; 



Geological changes, especially if involving change of climate ; 



Serious changes of climate ; 



Natural selection * ; 



Local adaptation (a species may have a peculiarity which is 

 useful in one country and valueless in another) ; 



Dying out of occasional old species ; 



Arrival of a species at its climatic limit ; 



Density of vegetation upon the ground at the time of arrival 

 of a species ; 



Presence or absence of mountain-chains in the land over 

 which the species has to travel in arriving ; 



Relative width of union between the country of departure 

 and that of arrival (the wider it is the more rapid may be 

 the spread of the species in the new country) ; and so on. 



It is recognised that this list is not complete, but it is 

 claimed that, although some of these causes probably come 

 into action in almost every case of any one species, the total 

 result is not a differentiating action, and the different effects 

 of these various modifying causes acting in so many different 

 directions cancel out when large numbers of species are dealt 

 with. The remaining effect after the cancellation is shown 

 to be due to some mechanical cause, such as age, which acts 

 on all species, genera and families alike, and which is, " at any 

 rate, independent of morphological and biological qualities." 



Willis founded his hypothesis on a large mass of statistics 

 of the estimated distribution of species in Ceylon (15), but has 

 confirmed it as a law by dealing with the measured distribu- 

 tion of species in New Zealand (17). At present, although 

 recent researches (19) extend the operation of the law to the 

 lower plants and (unpublished work) to some groups of ani- 

 mals, most of the facts are drawn from the known distribution 

 of the flowering plants, chiefly in Ceylon and New Zealand. 

 Dr. Willis's chief point lies in the emphasis he lays upon the 

 obvious commonplace that, as a rule, if a species is not dying 

 out, the longer it exists in any given country the wider will 



1 Natural selection enters to a large extent into the determination of the com- 

 monness of a species within its area of occupation (see 17, p. 448). 



