20 CONTACTS WITH DARWINISM I [ch. ii 



symmetry in its shoots as Podostemon, and as in that genus they 

 grow adventitiously from a creeping root. 



The plants of this family grow in conditions of uniformity that 

 can hardly be matched in any other flowering plants, but 

 amongst them is included the uniform action of a force which 

 cannot be escaped. Growing as they do, always upon smooth 

 water-worn rock, they cannot send their roots into the substratum, 

 so that the normal polarity of the young plant, which sends its 

 root down and its shoot up, is completely disturbed. By no con- 

 tortions can the plants grow normally, though the rock may be of 

 any kind of slope. 



There was no evidence to be found that would show that 

 natural selection had anything to do with the multiplicity of form 

 in these plants, for all were growing under the same conditions ; 

 but there was always this inescapable force urging dorsiventrality . 

 Under these circumstances, though he had started out with great 

 faith in adaptation and natural selection, the author became a 

 convert to the theory of mutational origin of species, adopting 

 from the very first the view that mutations or sudden steps might 

 at times be large enough to form species at one stroke. There were 

 no signs of real intermediates, yet surely here if anywhere they 

 might have been expected. An ordinary plant of another family, 

 growing more or less vertically upwards, would not usually come 

 under the continual influence of any powerful agent which would 

 tend to make its mutations go in any particular direction, but 

 with the Podostemaceae they were always being pushed in the 

 direction of dorsiventrality by the maximum force that nature 

 was capable of exercising in that direction. The mutations of 

 ordinary plants would give rise to specific differences in which 

 one could see no result of any particular directing force — there 

 was little or nothing to choose between them, and they were 

 morphological differences, with no adaptational value. In the 

 Podostemaceae, on the other hand, the mutations showed the 

 result of the continual force that was acting upon them, in a 

 dorsiventrality that on the whole tended to be continually more 

 and more marked the more local the genus might be. But it was 

 only an adaptation in the sense that moving restlessly in bed 

 might be described as adapting oneself to wearing pyjamas of 

 the fabric of which hairshirts were made. The dorsiventrality 

 was simply a morphological feature which had been forced upon 

 the plants. Upon this view, the difference in morphology between 

 the American and the Asiatic forms was also easily accounted for 



