6 INTRODUCTORY [tt. i 



effects of age only show clearly when one deals with mani/ 

 species, and those allied (and therefore more or less similar, both 

 in structure and reactions). 



I must consider myself very fortunate in having finally dis- 

 covered that the effects of age were sufficiently clear to be 

 arithmetically expressed. It is consequently possible now to 

 disentangle them to some extent from the effects of the other 

 factors acting upon distribution, and this should tend to make 

 the study of these other factors and their results an easier matter. 

 It seems to me by no means impossible that they too may prove 

 amenable to statistical treatment. Many biologists have a feeling 

 of dislike to the introduction into biology of the more exact 

 methods of arithmetic; as Hooker wrote, many years ago, "all 

 seem to dread the making botanical geography too exact a 

 science," But we have become accustomed to their use in the 

 study of genetics, and we may hope that their employment in 

 geographical work may not ultimately prove too repugnant. 



What has really surprised me in my Avork upon Age and Area 

 more than anything else, and what seems at the same time to 

 rouse some antagonism, is that the figures that have been given 

 in many papers, by myself and others, show such clear and un- 

 mistakable results that it is evident that mere age of species is 

 a much more important factor in geographical distribution than 

 we had been inclined to suppose. By the use of my hypothesis 

 that area occupied is largely dependent upon age. one can make 

 so many predictions about the geographical distribution of 

 plants, especially within comparatively small areas, and find 

 them correct Avithin such small limits, that it is evident that 

 mere age is a Axry important factor indeed, and consequently 

 that distribution, when one works with groups of species, and 

 OA'er enormous periods of time, is a much more mechanical pheno- 

 menon than we had been inclined to think. 



Of course age in itself cannot effect anything; what is really 

 meant is that the resultant effect of all the active factors, like 

 dispersal methods, etc., is so uniform, when one considers long 

 periods of time and takes an average of several allied species, 

 that these species spread indefinitely at a fairly steady average 

 rate. This rate, as I have pointed out in most of my papers, 

 will probably not be the same for any two species, but for allied 

 forms Mill not usually differ very much, so that by taking groups 

 of ten allies, and comparing with other groups allied to the first, 

 the rate of expansion of area will be a fair measure of age. 



