THE AGE AND AREA LAW 



A FUNDAMENTAL LAW OF GEOGRAPHICAL 



DISTRIBUTION 



By JAMES SMALL, M.Sc. (Lond.), Ph.C, 



Assistant Lecturer in Botany, Bedford College, and Lecturer in Botany to the Pharmaceutical 



Society of Great Britain 



A theory of geographical distribution has been brought 

 forward in a series of recent papers (10-19) by Dr. J. C. Willis, 

 which should radically change the outlook on many problems 

 of evolution. It has been expressed variously by the author 

 at different stages in the controversy which has arisen as a 

 result of his work. 



The law was first stated (15, p. 340) in the following form : 

 11 The commonness of a species depends upon its age from the 

 time of its arrival in, or evolution in, the country. 1 The com- 

 monness of any individual species will, of course, depend upon 

 its degree of adaptation to local conditions and upon many 

 things, such as the sudden appearance of new factors like 

 diseases, which can only be regarded as chance." As the 

 result of further study and some controversy, it was restated 

 later (17, p. 456) thus : " The geographical distribution of a 

 species (i.e. the area which it includes within its outer localities) 

 within a fairly uniform country not broken by serious barriers 

 depends upon the age of that species within that country." 

 As the result of further controversy, the law is now stated 

 (18, p. 206) to be subject to the qualifying phrase " so long as 

 conditions remain constant," and the following causes are 

 enumerated (loc. cit.) which may modify the operation of the 

 fundamental principle of age and area : 



Chance (the operation of causes as yet not understood) ; 

 Action of man in opening up a country, cutting of forest, 

 exploring, making fires, etc., etc. ; 



1 As this applies to the average species, and not necessarily to any particular 

 one, the species must be taken in groups of about 20 (cp. 15, p. 325). 



439 



