CH. II] THE PODOSTEMACEAE 13 



variation was required, such as was supplied by de Vries' theory 

 of mutation (48). 



Evolution by gradual variation thus has many difficulties in 

 its path, which in the first enthusiasm of natural selection were 

 passed over with little notice. Under the influence of the criticism 

 of Fleeming Jenkin, it had to be admitted that all the new plants 

 of a considerable area must vary more or less in the same direc- 

 tion to prevent the new variation from being lost by crossing. It 

 would be lost at the edge of its territory, but would presumably 

 survive in the middle. The area of the parent species would thus 

 tend to become more or less discontinuous. It had to be assumed 

 that the parent did not vary in a favourable direction also, but 

 as all variation was assumed to be structural (it could hardly 

 be otherwise, as natural selection was trying to explain a 

 structural evolution), it was easy to suppose that the parent could 

 not vary in such a way. It also had to be assumed that the con- 

 ditions continued to change for a very long time, to such an 

 extent anyhow as to pass the sterility line, or a new species could 

 not be formed. This new species would evidently be well adapted 

 to the new conditions whose existence was responsible for its 

 coming into being, but it had also to be assumed that when 

 formed, or partly formed, it would then prove so suited to the 

 region in which the parent was still supreme as to kill out the 

 latter there also. This was a pure assumption, but was necessary 

 in order to explain the spread of the newer and better-adapted 

 species, which in turn was to explain their wide distribution. We 

 have shown in Age and Area, p. 34, that the older species will 

 probably gain continually upon the younger in rate of dispersal, 

 supposing, which seems to be the case, that there is no reason 

 (when they are taken in groups) why one should spread more 

 rapidly than another nearly related to it. If the area to which the 

 new species was ultimately to reach were very large, it was really 

 rather absurd to talk of it as adapted to the whole area. It must 

 have been just a case of luck that it proved so sufficiently suited 

 to far-away places as to be able to establish itself there, though 

 once arrived it would begin to suit itself in detail to the local 

 conditions. And it must not be forgotten that early species would 

 have the best chance both of rapid travel and easy settlement. 



Finally, among the difficulties of Darwinism, it was evident 

 that the variations must be such that natural selection could 

 work upon them when they did appear, and as to that we have 

 but little evidence. 



