CH. vii] ISOLATION 63 



specially adapted to the exact local conditions in which it is 

 found, except by the internal, physiological adaptation that must 

 always be going on. It would be killed out at birth if not reason- 

 ably well suited to the local conditions. 



Turrill (47) has shown that in the Balkans one may find pairs 

 of altitudinally differing species like Bellis longifolia and sylvatica, 

 a fact which affords further evidence in favour of the author's 

 view that isolation and elevation, one or both, may lead to the 

 formation of endemic species upon mountains. In Ceylon, of the 

 sixty-two genera represented by endemic species upon mountain 

 tops, forty-three also have endemics at lower elevations, and only 

 nineteen have not, a fact which makes the supposition that 

 those of high levels are relics seem a little far-fetched. 



The Podostemaceae as a family are very isolated, and they 

 grow submerged in water, usually at what are only moderate 

 elevations, yet they have many species. Though isolated from 

 other plants, they usually cover their own habitat, the rocks, 

 fairly thickly, so that one hesitates to suggest that they would 

 have so many species were they really isolated as individuals. It 

 would seem more likely in their case that they owe their numbers 

 to the overhead force of plagiotropism that is always at work 

 upon them. There are probably quite a number of causes that 

 may lead to the formation of new species. 



Lakes formed by elevation of the coast of the Black Sea con- 

 tain, I am assured, endemic species of cockles, a fact which would 

 seem to favour isolation, especially as they are close to sea-level. 



Siparuna (p. 35) has the great bulk of its local species in the 

 mountains rather than in the plains, and the same is the case with 

 many other genera, whilst many of the isolated islands that 

 contain so many endemics are also mountainous. These facts 

 might seem in favour of elevation (cosmic rays) rather than isola- 

 tion, but other plants, such as the Dipterocarps, show many 

 endemic species in the plains, usually in dense forest. Here 

 species formation is probably connected with age. 



Probably both isolation and elevation may be potent causes 

 leading to well-marked development of new species. In the 

 former case mutation is quite probably due to slow gene change, 

 as Harland has suggested, but this would probably bring about 

 sudden mutation by the adding up of strains until they became 

 so strong as to cause some sudden kaleidoscopic change. In the 

 latter case, if the cause of mutation be some effect of the bom- 

 bardment of the genes by cosmic rays, one might expect the 



