CH. II] THE PODOSTEMACEAE 21 



by some small difference in morphology between the first parents 

 in the two countries, which had the effect of urging the first 

 mutations in somewhat different directions. It is, of course, true 

 that natural selection might do the same with the same start, but 

 it is not quite so easy to imagine. 



Any member of the family seems to be able to live without anv 

 great difficulty where any other member can live (53, p. 535) 

 though probably they have some preference as to speed of water, 

 and one must remember that in any case this varies with the 

 level of the river, being usually faster the higher the level. 

 People who came with me to look at the Podostemaceae growing 

 in the river near Peradeniya, when they saw the flat, closely 

 adherent Lazvias or Hydrohryums, used to say "obvious adapta- 

 tions to escape being carried away by the fast water". But in 

 Brazil the comparatively enormous Moureras and other forms, 

 3 or 4 ft. long, yet attached only at one end, lived in water that 

 was going at twice the speed of that in Ceylon. 



Much or most of the evolution that had gone on, therefore, 

 seemed to be completely de luxe, for there was no need for the 

 new forms, nor was there any adaptational niche that would suit 

 one form onlv. and not also many others. It would almost seem 

 as if, in cases like this, if not perhaps in most, evolution must go 

 on, whether there be any adaptational reason for it, or not. 



The explanation of the distribution of the Podostemaceae, 

 as given by current theories based upon natural selection, en- 

 counters some awkward difficulties. The most highly dorsiventral 

 forms are the most local, i.e. they are "the relics of previous 

 vegetation, defeated by the more widely distributed ones". In 

 other words, the family began with extreme dorsiventrality, and 

 then, so to speak, repented of it to some extent. But to become 

 less dorsiventral under the constant and utmost influence of a 

 force that is urging movement in the opposite direction, can 

 hardly be looked upon as likely to happen under the influence of 

 natural selection and the whole situation becomes an impasse. 



The phenomena sho^^^l by the Podostemaceae are almost 

 exactlv matched in the allied family Tristichaceae, which has 

 much the same distribution, and are also matched by the pheno- 

 mena shown by the most completely parasitic plants, such as 

 Rafflesiaceae or many fungi, which, though they grow in mar- 

 vellously uniform conditions, none the less show important 

 structural differences. 



The universality of this type of distribution, with the more 



