550 Lack of Adaptation in the Tristichaeece and Podostemacece. 



numerous species of these families ? And why are there any gaps between 

 the species ? Wliy are they not filled up by intermediate forms, only differing 

 from one another in very slight degree ? (If gaps once arise between species, 

 larger ones will inevitably, so far as one can see, arise between genera, so that 

 we may leave the genera out of account.) Does this mean that mutations 

 may be larger than we usually imagine ? Or does it mean that a mutation 

 in one direction involves further mutation in the same direction ? Or what 

 does it mean ? 



It would scarcely seem, perhaps, as if the actual directioii of mutation could 

 have been selected by the forces acting on these plants, unless in the 

 progenitors there appeared several mutations in different directions involving- 

 others in the same directions, all of which were selected. 



As one can almost say that no variations other than dorsiventral ones or 

 variations in the direction of longer or more divided leaves or thalli, have been 

 perpetuated, and as the dorsiventrality is no advantage, and as there is no 

 natural selection except in the direction of extermination, it is very difficult 

 to escape the conclusion that the evolution was more or less guided in a 

 definite direction by the plagiotropism. Perhaps this force or natural 

 selection exterminated variations, other than slight, in other directions. 



On the whole, we are inclined to think, though as yet with great diffidence, 

 and with an open mind, that the evolution of these families was by indis- 

 criminate mutation, or mutation in every direction, without natural selection, 

 the mutations in the direction of dorsiventrality and perhaps in the direction 

 of longer and more divided leaves being on the whole more easily perpetuated 

 than others — many of which would be killed out by natural selection — and 

 this on account of the permanent deflecting forces acting on these plants, 

 and which we may perhaps call their evolution-factors. A strongly marked 

 evolution-factor, like plagiotropism in these families, can compel evolution to 

 move on the whole in a definite direction, without any reference to the 

 advantages or disadvantages to be derived from so moving. 



To accept this result as general will explain without any difficulty the 

 presence of such countless numbers of useless characters in plants* and may 

 help to account for the great changes in the botanical landscape which seem 

 often to accompany the greater changes in the geological landscape — the 

 great change of conditions on getting into plagiotropic water life seems 

 to have produced the great variety in the plants we have been considering, 

 and perhaps the greater landscape changes were similarly accompanied by 

 great changes of conditions. 



* Of. De Vries, ' The Mutation Theory,' vol. 1, p. 208 (Delbceuf), 1912. 



