CM- iJ INTRODUCTORY 3 



newed and extraordhiary vigour, whilst the mechanical factors, 

 except perhaps the purely negative influences of barriers, were 

 left comparatively neglected. For many years there was re- 

 markable progress in our knowledge of geographical distribution, 

 but this has now all but ceased, except in regard to the study of 

 the purely local distribution of species in reference to the purely 

 local changes of the different factors of climate, water-supply, 

 associations of plants covering the ground, and the like, in 

 which direction much work of extreme value is being carried' on. 

 But in regard to the wider general distribution of plants about 

 the globe, we seem to have arrived at a period when the limiting 

 factor, to use Blackman's words, has become the lack of a satis*^ 

 factory theoretical background, which will provide efficient 

 working hypotheses for the conduct of investigations that shall 

 lead to real advances in our knowledge of the fascinating sub- 

 ject of geographical distribution. I have myself heard a leading 

 authority upon this subject say that he thought that it was 

 almost beyond the range of human capacity. 



In this emphasising of the effects of the vital factors, the 

 action of mere age, which must evidently be of some importance, 

 has been more and more lost to view. And yet in 1853 Lyell 

 (69, p. 702) wrote 



As a general rule, however, species common to many distant 

 provinces, or those noAv found to inhabit very distant parts of 

 the globe, are to be regarded as the most ancient. Numericallv 

 speaking, they may not perhaps be largelv represented, but their 

 wide diffusion shows that they have had a long time to spread 

 themselves, and have been able to survive many important 

 revolutions in physical geography. 



Again he says 



Nor do I doubt that if very considerable periods of equal 

 duration could be compared with one another, the rate of change 

 in the living... world might be nearly uniform. 



And yet again 



Every local revolution... tends to circumscribe the range of 

 some species, while it enlarges that of others ; and if we are led 

 to infer that new species originate in one spot only, each must 

 require time to diffuse itself^ over a wide area. It will follow, 

 therefore, from the adoption of this hypothesis, that the recent 

 origin of some species, and the high antiquity of others, are 

 equally consistent with the general fact of their limited dis- 

 tribution ; some being local, because they have not existed long 

 enough to admit of their wide dissemination; others, because 



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