CH. xiv] GENERAL DISCUSSION 181 



structural characters, in which one can neither find nor imagine 

 any adaptational value whatever. 



If differentiation be accepted, the process of evolution may be 

 quickened up considerably, for a single mutation may effect in 

 one step a change which might take an immense time under the 

 action of natural selection, especially when one fully realises that 

 the vast bulk of structural differences have no adaptational value. 

 And if, as upon this view would seem highly probable, mutations 

 were, on the whole, larger as one went further back into past 

 times, the difficulties of explaining the origin of great groups like 

 the ferns will be greatly lessened. It must not be forgotten that 

 these also must be explained by natural selection, which as yet 

 has shown itself quite incompetent in this respect. So long as we 

 try to explain these by adaptational changes, or by dying out of 

 transitional stages, so long shall we be in great difficulty. The 

 theory of divergent mutation requires nothing of this kind, and its 

 capacity of explanation is far greater than is that of the theory of 

 gradual adaptation. It seems to the writer that the theory of 

 natural selection leads to too many untenable positions to be any 

 longer acceptable, and that differentiation, working downwards 

 towards the species, and by large mutations, diminishing as one 

 comes downwards (on the whole) should take its place. 



The evidence is clearly in favour of differentiation, or diver- 

 gent mutation, rather than natural selection. The largest and 

 most divergent mutation gives rise on the whole to the family, 

 while the later and usually less divergent ones give rise to the 

 later genera and species, which come as a rule within the limits 

 marked out by the first. This agrees completely with the familiar 

 fact that the key to a family can be so easily made upon these 

 lines, with the largest differences coming first, followed by 

 smaller and smaller ones down to the specific and varietal dif- 

 ferences at the bottom. But this feature is a matter of extra- 

 ordinary difficulty to explain upon the Darwinian theory, under 

 which two species form by progress in gradual adaptation in 

 slightly different directions, the unmodified and the transitional 

 forms being killed out, until at last the difference is so great that 

 they have become new species. But if a slight variation in a 

 favourable direction is enough to give an advantage over the 

 forms that have not varied, what is to be gained by going on with 

 the variation until it becomes specific, and how is this to be done? 

 What adaptational need made one species adopt an alternate 

 leaf with a phyllotaxy of 5/8, its nearest relative opposite leaves? 



