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’, 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 
“émigrant pour suivre une direction déterminée et marcher du nord 
au sud, 4 la recherche de régions et de stations plus favorables, mieux 
appropriées aux adaptations acquises, 4 méme que la température 
terrestre perd ses conditions premiéres®.” If, as is so often the case, 
the theory now seems to be @ 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 in Appendix G, p. 438, in Journal of a Tour in Morocco and the Great Atlas, 
J. D. Hooker and J. Ball, London, 1878. 
2 Origine Paléontologique des arbres, Paris, 1888, p. 28. 
