CH. VI] ADAPTATION 59 



The mere fact that the prominent genera that occupy any kind 

 of marked ecological standpoint, such as a bog, a saltmarsh, a 

 mountain, a chalk-down, are usually the large and widespread 

 genera, is enough to show that there was but little selection — 

 they were the oldest and got there first, and being adaptable they 

 became functionally modified to suit their new surroundings. 



What has been said about gradual adaptation applies equally 

 to the view at present rather in favour that mutations were 

 small, and that selection presently resulted in another small step, 

 and so on. But what is to ensure that a small step in one direction 

 shall be followed by a second, or that conditions shall continue 

 to change in such a way as to make it worth while for such a 

 thing to occur? 



The balance of probability would seem to be in favour of the 

 appearance of structural characters by single mutations, and in 

 that case it seems rather absurd to talk about adaptations in 

 them. The adaptation is rather the internal and functional 

 adaptation. 



It would seem quite possible that climatic conditions all over 

 the world have been gradually differentiating and becoming more 

 varied as time has passed. On the whole, they have almost 

 certainly become drier, though probably not in such places as 

 many coastal regions. This would affect newly formed species by 

 gradual^ restricting their freedom of movement, or even by 

 forming impassable barriers. To move in a region of more or less 

 uniform climate would probably require comparatively little of 

 fresh adaptation to each new habitat, but if the climate were 

 changing from one place to another, this adaptation would have 

 to be greater, and would presumably need more time. This would 

 in turn make the rate of travel slower, and it is quite possible 

 that the change of climate might, so to speak, pass it upon the 

 way, and erect a barrier some distance in front, the species 

 reaching the limit of possible acclimatisation. This would seem 

 to have happened in Ceylon, for example, where the island is 

 rather sharply marked out into dry and wet zones. Comparatively 

 few species are found on both sides of the divide, and really 

 frequent in both zones. Many genera show a number of species 

 in the wet zone with few in the dry, others the reverse, whilst of 

 the genera that are confined to one zone, most occur in the wet. 



It is clear that it is somewhat stretching a point to say that 

 new genera, arising locally, as we have seen will in all probability 

 be the case, are adapted to wide spread over the world. As only 



