ESSAY-REVIEW 



THE DYNAMICS OP DISTRIBUTION. By A. G. Thacker, 

 A.R.C.S., being a review of : AgeandArea. By J.C. Willis, Sc.D., F.R.S. 

 [Pp. X + 259.] (Cambridge University Press, 1922. Price 14s. net.) 



The sub-title of this book is : "A Study in Geographical Distribution and 

 Origin of Species." Dr. Willis was for many years Director of the Botanic 

 Gardens at Kandy, Ceylon, and he subsequently held a similar position at 

 Rio de Janeiro. He has made most detailed studies of the distribution of 

 plants, especially of those found in tropical and southern countries. He has 

 collected laboriously a mass of most interesting statistics, and he claims to 

 have discovered a fundamental principle which he believes underlies the 

 geographical distribution not only of plants but also of animals. It is this 

 principle which he has named " Age and Area." The hypothesis is simple. 

 Take a group of aUied species and find the average area covered by them. 

 Then take another group of allied species, fairly closely related to the first 

 group, and find the average area covered by them. The two average areas 

 are then believed to indicate the average ages of the two groups, that is, the 

 relative ages, not any absolute figures, of course. If the average area oif the 

 second group be greater than that of the first group, then it is inferred that 

 the average antiquity of the second lot of species will also be greater than 

 that of the first lot. The principle is not to be applied to individual cases ; 

 and it must not be applied to groups of species which are not fairly closely 

 allied. But with these qualifications (the necessity of which is, of course, 

 obvious) age and area are conceived to vary together. The hypothesis may, 

 however, be given in Dr. Willis's own words, his definition (p. 63) being as 

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

 any 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 countrj'-, but may be enormously modified 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 like, also by the action 

 of man and other causes." 



It will be noticed that this definition is well hedged with qualifications ; 

 but in point of fact, unless the author's statistics are susceptible of another 

 explanation, there is singularly little sign of the " enormous modifications." 

 The book is divided into two parts. The first part is entitled " The Present 

 Position of Age and Area," and contains nine chapters, of which five are more 

 or less introductory. Chapters VI and VII constitute the core of the book. 

 They describe the proofs of the hypothesis and give the supposed " con- 

 firmation by prediction." Part II is entitled, " The Application of Age and 

 Area to the Flora of the World, and its Implications," and has thirteen chapters, 

 of which four are contributed by other writers, namely. Dr. H. B. Guppy, 

 Prof. J. Small, Mrs. E. M. Reid, and Dr. Hugo de Vries. The most important 

 chapter in Part II is, however, that entitled " Size and Space." The principle 

 there enunciated is really an extension of the idea of age and area. By 

 " size and space " the author means that a large genus (a genus with many 



474 



