756 ^ CENTURY OF PROGRESS IN THE NATURAL SCIENCES 



cumstances was certainly the normal swing of the pendulum, in this ease from 

 the extreme of formal taxonomy and purely descriptive ecology. Two other not 

 clearly related influences, however, were more particularly concerned. These 

 were the publication by Willis (1922) of his theory of age and area and the 

 publication by "Wegener (1924) of his theory of continental displacement. 



There is much evidence for the belief that the success of a scientific theory 

 has often been due to the fact that a general combination of trends and circum- 

 stances, not in themselves easily discernible, have served to predispose public 

 opinion favorably toward it, almost as if an unconscious sort of propaganda 

 had been at work. There is little doubt, for instance, that this is broadly true of 

 Darwinism itself, which, when the time was fully ripe, became widely estab- 

 lished with what was really remarkable rapidity and unanimity. So with the 

 two theories just mentioned. They were something new when the times were 

 set for novelty and because of this, and also perhaps because each contained an 

 element of the mysterious, they gained considerable attention. 



Willis' work appeared straightforwardly as a contribution to botanical 

 thought, but it developed from an evolutionary approach to the subject. Readers 

 may find elsewhere (e.g., New Phytol. [1951] , 50 : 135) accounts of it longer than 

 is appropriate here. It need only be said that Willis was attracted to the subject 

 of plant geography mainly because of the way in which it seemed to him capable 

 of helping toward a better understanding of the processes of organic evolution 

 and particularly because of the way it might be made to afford evidence against 

 the theory of natural selection, which Willis' experience caused him to criticize. 

 Very briefly, Willis maintained that the choice lay between natural selection 

 and mutation and that, since mutation requires no assumption of a widespread 

 supersession and elimination of "unfit" species, which is inherent in the con- 

 ception of natural selection, any detection of an exponential rate of speciation 

 and spread might be held to indicate that mutation rather than selection had been 

 the paramount process in evolution. Such an exponential rate Willis claimed to 

 demonstrate in the "hollow curve" type of graph. As a projection of this, as it 

 were, Willis argued that under continuous mutation, not only would the totality 

 of species constantly increase, but, barring accidents, the longer a species existed 

 the wider would be its spread, and it is by this conception of "age and area," 

 as he called it, that his scientific work is most familiar. 



This is not the place to attempt an appraisal of Willis' theories or an assess- 

 ment of his direct contribution to plant geography as such. His claim to a place 

 in the history of that subject is based on something rather different, for the 

 service that he rendered was that of provoking (to use the ynot juste) a renewed 

 interest in the whole science. But this does not altogether explain the almost 

 violent reaction that many of his opinions occasioned and there were, it seems, 

 two other reasons. One certainly was that he had the temerity to question the 

 long popular theory of natural selection; the other, that he puts into words 

 what many felt. For both these different reasons his writings received a measure 

 of publicity and criticism which possibly surprised no one more than their aiithor. 



As we now look back through the years, the nature of Willis' achievement in 

 plant geography has become clearer. It is that he showed, even if without first 

 intent, that plant geography was not the exhausted subject which had yielded 

 place to more modern disciplines but was a living one which still posed pro- 



