2 INTRODUCTORY [pt. i 



upon the latter; now it is closed, or closed to all but a few whose 

 seeds may be carried, by wind, birds, or man, across the dividing 

 seas. In Tertiary times, Europe was covered with forest in which 

 grew many things not now found there ; the onset of the cold of 

 the glacial period, and the secular changes of climate, have so 

 altered the conditions that the Tertiary forest has disappeared. 



So complicated is the interaction of all these many factors, 

 and their continual changes, that in general it has been con- 

 sidered impossible to say why a given plant should be found to 

 occupy a given area, while another species of the same genus 

 occupies one much larger or much smaller, though it may look 

 almost exactly like the first, and may differ from it only in 

 characters to which we cannot, without great stretch of the 

 imagination, attach any serious importance for life or success. 

 We have been luiable to say why, for example, Coleus barbatus 

 should be found almost over tropical Asia and Africa, while 

 C. elongatus, which differs chiefly in the form of the calyx and 

 of the inflorescence, is confined to the summit of one mountain. 



For sixty years we have been under the wonderful fascination 

 of the theory of evolution by means of infinitesimal variations, 

 or minute changes of character from individual to individual. 

 At first, and for a long period, this theory seemed to be capable 

 of explaining almost everything, and to it we owe what could 

 perhaps have come in no other way, the establishment of the 

 doctrine of evolution, now universally adopted, but which until 

 the latter part of the last century, though 2000 years old, had 

 met with no acceptance. To quote Huxley (22 in List of Litera- 

 ture, II, pp. 180, 197), "To any one who studies the signs of the 

 times, the emergence of the ])hilosophy of Evolution, in the 

 attitude of claimant to the throne of the world of thought, from 

 the limbo of hated and, as many hoped, forgotten things, is the 

 most portentous event of the nineteenth century." "...the pub- 

 lication... had the effect... of the flash of light, w^hich to a man 

 who has lost himself in a dark night, suddenly reveals a road 

 which, whether it takes him straight home or not, certainly goes 

 his way." 



Under the glamour of this theory, the tendency naturally was 

 to lay the greatest stress upon the vital factors in distribution, 

 for these were the only ones which could differ from individual to 

 individual, or from species to species. The means of dispersal 

 open to plants, their reactions to the climate, etc., and their 

 adaptations to various ends, were therefore studied with re- 



