Chapter III — 41 — On Areas 



that ancient species be called "senior species" and young species — 

 "junior species". 



These citations fully suffice to show that the significance of age was 

 never lost sight of by botanical geographers, although, no doubt, as 

 compared with other biological factors, it was given too little attention 

 and its importance underrated. 



Age — or the length of time during which the dispersal of species 

 and, hence, also the formation of floras have taicen place — is for 

 historical plant geography a factor of just such prime importance as 

 the duration of geological periods established by Lyell was for the 

 theory of evolution. Just as a necessary premise for the evolution of 

 organisms is the duration of time taken by the latter for their develop- 

 ment, so all the regularities in historical biogeography may be under- 

 stood only by taking into account the length of time during which 

 they have existed. 



Consequently, we cannot fail to give serious consideration to 

 Willis's treatise on "Age and Area" (1922), devoted to a study of it 

 as a botanico-geographical factor and constituting a summary of his 

 many investigations on this problem begun in 1907. The works of 

 Willis are not mere armchair theorizing but are based on twenty 

 years of field work devoted to the geographical study of plants in na- 

 ture, in tropical South America and particularly in tropical Asia, where 

 for a long time he was director of the Botanical Garden at Ceylon. 

 He verified his conclusions by comparison with the data published by 

 him in his "Flora of Ceylon" and other floristic works. At the same 

 time, however, the very fact that he used the floras of these tropical 

 regions as the basis for his conclusions constitutes the cause of the 

 onesidedness of his conclusions, on account of which numerous criti- 

 cisms were directed against him. The floras of the tropical regions of 

 America and Asia are the only floras on the globe that since the 

 Cretaceous period have not been subjected to great climatic changes. 

 Consequently, laws established with respect to the formation of areas 

 of species of these floras are applicable only to such floras as are 

 characterized by unhampered development. They are not of uni- 

 versal significance and cannot be applied to all the floras of the earth. 

 The criticisms of Willis's book were directed chiefly on this flaw in 

 the propositions advanced by him. 



Studying the flora of Ceylon, Willis was struck by the great 

 differences in the size of areas occupied by different species of the same 

 genus, some of which were endemic to the island and others not. 

 This led him to the conclusion that "the endemic species occupied, on 

 the average, the smallest areas in the island, those found also in Penin- 

 sular India (but not beyond) areas rather larger, and those that ranged 

 beyond the peninsula the largest areas of all (again on the average) " 

 (Willis, 1922, p. 65). At the same time, the number of species in each 

 class was found to vary, increasing or diminishing depending on the 

 size of the area. This may be clearly illustrated by his data on the 

 flora of New Zealand. Taking the extent of the areas of species in 

 this flora along the north and south diameter of the island, the follow- 

 ing gradations are obtained (ibid., p. 64): — 



