AGE AND AREA 



A REVIEW OF J. C. WILLIS' THEORY OF THE ORIGIN OF SPECIES 



Hugo de Vries 

 Lmitcrcii, Holland 



IN a series of articles, published 

 during the last twenty years, J. C. 

 Willis has tried to show that the 

 dispersal of plants is governed by gen- 

 eral laws and independent of so-called 

 adaptations, or of any kind of advan- 

 tageous response to local conditions. 

 The main factor of distribution is age, 

 a cause which works in the same man- 

 ner on all plants. The rate of expan- 

 sion of area is a fair measure of age. 

 The most widely distributed genera 

 and species in every family are to be 

 considered as the oldest within that 

 group, and, taken broadly, the remain- 

 ing ones must be progressively young- 

 er, the smaller the area they occupy. 

 Number of species usually goes parallel 

 to geographical area, and therefore the 

 genera which are richest in species will 

 also, as a rule, be the oldest of their 

 group. Senecio, with 1500 species, has 

 been the source whence the family of 

 the Compositae has been developed. 

 Astragalus with 1600 species is at the 

 bottom of the papilionaceous plants and 

 other large genera could easily be 

 added. 



Of course, age in itself is not the 

 cause of distribution ; it only allows 

 the time for this process. Moreover, 

 when one considers single species, as 

 has been done most generally in plant- 

 geography up to this time, the influence 

 of adaptations, of dispersal methods 

 and so forth may easily lead to the con- 

 ception of special laws for different 

 forms. Therefore it is necessary to 

 study averages, instead of single cases. 

 In doing so it is found that the result- 

 ant effect of the active factors, bio- 

 logical as well as ecological, is very 

 uniform and almost wholly the same 

 for different families or for the mem- 



bers of different floras. Considering 

 long periods of time and taking the 

 species in small groups of allied forms 

 (e. g. of 10-12 each), in order to cal- 

 culate averages, one finds that all plants 

 spread at a fairly average rate. 



The author has now collected his 

 results in a book, entitled Age and 

 Area, A Study in Geographical Dis- 

 tribution and Origin of Species (Cam- 

 bridge, 1922, p.p. 259). The discus- 

 sion of his law is founded here upon 

 a broad basis, combining the results 

 of the investigation of numerous local 

 floras and comparing them on all sides 

 with previous views. Many objections 

 and many criticisms had already been 

 published by different authors and 

 they are now dealt with in such a way 

 as to insure the validity and the general 

 applicability of the main law. Confirm- 

 ations have also been numerous and 

 among these I might cite the work of 

 J. Small upon the evolution of the 

 Compositae. In a chapter on this 

 group, inserted in Willis' book (pp. 

 1 1 8- 1 36), Small shows that the pedi- 

 gree of this family, as derived from 

 the morphological characters of the 

 stigma and the stamens and from the 

 study of the fossil fruits found in the 

 deposits of the tertiary period, wholly 

 agrees with the predictions which can 

 be made from Willis' law. Expansion 

 of area, and number of species, if cal- 

 culated in averages for the different 

 tribes and subtribes. are almost exactly 

 parallel to the ages derived for them 

 from systematical and palaeontological 

 data. 



The Flora of Ceylon 



Willis started his statistical researches 

 with a study of the flora of Ceylon, 

 and afterwards applied his results to 



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