316 Geographical Distribution of Plants 



fresh illustrations. A few years later Asa Gray found the explanation 

 in both areas being centres of preservation of the Cretaceous flora 

 from a common origin. It is interesting to note that the paper in 

 which this was enunciated at once established his reputation. 



In Europe the latitudinal range of the great mountain chains 

 gave the Miocene flora no chance of escape during the Glacial period, 

 and the Mediterranean appears to have equally intercepted the flow 

 of alpine plants to the Atlas 1 . In Southern Europe the myrtle, the 

 laurel, the fig and the dwarf-palm are the sole representatives of as 

 many great tropical families. Another great tropical family, the Gesne- 

 raceae has left single representatives from the Pyrenees to the Balkans ; 

 and in the former a diminutive yam still lingers. These are only 

 illustrations of the evidence which constantly accumulates and which 

 finds no rational explanation except that which Darwin has given 

 to it. 



The theory of southward migration is the key to the interpretation 

 of the geographical distribution of plants. It derived enormous 

 support from the researches of Heer and has now become an accepted 

 commonplace. Saporta in 1888 described the vegetable kingdom as 

 " Emigrant pour suivre une direction determined et marcher du nord 

 au sud, k la recherche de regions et de stations plus favorables, mieux 

 appropri^es aux adaptations acquises, & mme que la temperature 

 terrestre perd ses conditions premieres 2 ." If, as is so often the case, 

 the theory now seems to be ct priori inevitable, the historian of 

 science will not omit to record that the first germ sprang from the 

 brain of Darwin. 



In attempting this sketch of Darwin's influence on Geographical 

 Distribution, I have found it impossible to treat it from an external 

 point of view. His interest in it was unflagging; all I could say 

 became necessarily a record of that interest and could not be detached 

 from it. He was in more or less intimate touch with everyone who 

 was working at it. In reading the letters we move amongst great 

 names. With an extraordinary charm of persuasive correspondence 

 he was constantly suggesting, criticising and stimulating. It is 

 hardly an exaggeration to say that from the quiet of his study at 

 Down he was founding and directing a wide-world school 



1 John Ball iu Appendix G, p. 438, in Journal of a Tour in Morocco and the Great Atlas, 

 3. D. Hooker and J. Ball, London, 1878. 



2 Origine Paleontologique des arbres, Paris, 1888, p. 28. 



