CH. xiii] D. GEOGRAPHICAL DISTRIBUTION 153 



show the absurdity of trying to draw a definite line of distinction 

 between endemic and non-endemic. 



Working upon the theories of Age and Area and of Differentia- 

 tion, this distribution is exactly what one would expect to find, 

 but it is extremely difficult to account for upon the theory of 

 natural selection or of gradual adaptation. On that theory the 

 widely dispersed things are supposed to be the best adapted. But 

 to what? It is clear that if the distribution is very wide, each 

 individual or group of individuals found in any small region can 

 only be adapted to that region. Suitability to other regions that 

 differed to some extent from the first could not be such an advan- 

 tage to a species that it would help it to settle in the first region. 

 Natural selection, picking out species suitable to A, would not at 

 the same time pick out qualities that would suit the species 

 to B ; it could not even know, to put it in a kind of personal way, 

 that B existed, and that A would gain in area of distribution by 

 being able to settle there without further adaptation. A species 

 must become adapted in turn to every change of conditions with 

 which it may meet, whether differing soil, temperature, moisture, 

 or biological conditions, and so on, and when at last it meets with 

 conditions that go beyond its possible range of adaptation, then 

 it will have met one of the boundaries that limit distribution, 

 already fully enough described in Age and Area. Probably there 

 is some kind of limit to adaptation (or it may be only to speed of 

 adaptation) in most or all species. Sooner or later they will come 

 up against a barrier, most often probably climatic, which they 

 cannot pass. But at the meeting place of such barriers, e.g. in 

 Ceylon at the junction of the dry with the wet zones, one not 

 infrequently finds different species of the same genus, some on 

 one side, some on the other. This is apt to suggest that at some 

 time and place, one or the other species was becoming adapted to 

 one or the other zone, and that some kind of turn of the kaleido- 

 scope took place which resulted in the formation of the second 

 species, better adapted to the new conditions, though its morpho- 

 logical differences probably had nothing to do with physiological 

 problems, but were perhaps in some way a correlation. 



The general evidence of contour maps, of which a very good 

 example {Beta) may be found in Nat. Pflanzenfamilien, 2nd ed. 

 vol. XVI c, 1934, p. 461, is entirely in favour of differentiation and 

 age and area. It is sometimes suggested that at the centre of a 

 contour map the conditions are more varied, but very little 

 thought is required to show the absurdity of this contention. The 



