570 Ridley . — On Endemism and the Mutation Theory. 
the evolution in Hieracium or Rubus , nor even to the Grewias of the Malay 
region or the Pandani of Madagascar. 
Again, in Europe we have of Gesneraceae, Ramondia pyrenaica , Lam. 
(Pyrenees), R. serbica, Pane. (Serbia), R. Heldreichii , Boiss. (Thessaly), 
Haber lea rhodopensis , Friv. (Thrace). 
These grow in much the same kind of locality as the Saxifrages do, 
but what a contrast ! Of Saxifrages in Europe we have over a hundred 
species and innumerable forms. The nearest Gesneraceae to our four 
European species are in tropical Africa 1 and the Himalayas, while the 
Saxifrages range over the whole of the north temperate zone and even 
South Africa, Australasia, and South America. Now look at the Gesnera- 
ceae of the East Asiatic tropics ; of Didymocarpus we have over fifty species 
in the Malay Peninsula alone ; the closely allied Chirita , 7, Loxocarpus , 4, 
Paraboea , 13, Boea , 12. We find a large number more of Didymocarpus in 
India, Borneo, Java, and Sumatra ; Chiritas, about fifty more in Cochin 
China, Assam, Ceylon, Java; Loxocarpus , several in Borneo ; Paraboea in 
Siam and Assam ; Boea in Borneo, Java, Siam, and Cambodia. All these 
countries are in actual land contact with the Malay Peninsula, or were 
formerly so. They differ much in climate and soils. 
Let us assume that a species of Didymocarpus has been evolved in the 
Malay region, and spread gradually as far as the countries and localities 
would admit of its growing. In various spots, and perhaps distant parts 
of these countries, mutations formed, and became fixed mutations or what 
we should call species. A species is formed, let us say, on the Tenasserim 
limestone rocks ; it pushes down along the limestone chain again to the 
Malay Peninsula. Here it meets with allied species with which it can 
cross, and a fresh series of mutations is formed, some of which, adapted 
to special circumstances, can live and thrive on spots where the original 
species could not. 
We have the European Gesneraceae and Dioscoreaceae, and Aphyl- 
lanthes , all isolated species, with no relatives near enough to hybridize and 
cause the production of an extensive evolution. 
We have Saxifraga in Central Europe, in Arctic Europe, the whole of 
the temperate regions of Asia and America. The wave of cold in the 
glacial period must have driven out many species in Europe, but there were 
mutations which had adapted themselves to the cold and survived ; and 
these returning hybridized with the species surviving in the warmer valleys 
of the south and started a new series of mutations. Hieracium , Rubus , 
Rosa have all the same distribution and story. 
But the European Gesneraceae and Dioscoreaceae and Sowerbieae 
( Aphyllanthes ), the relics of an old long-lost flora, isolated by thousands of 
miles from any allied species, remain unaltered, with no series of mutations. 
1 S ain t p aul ia. 
