CHAPTER V 



CONTACTS WITH DARWINISM, continued. 



MUTATION 



X HE coming of mutation was mentioned above (p. 14) and it 

 was pointed out that it seemed to get over some, at any rate, of 

 the difficulties inherent in the employment of gradual variation. 

 In particular, as the new form was qualitatively, and not merely 

 quantitatively different from the old, the change was differen- 

 tiating. Further, it was practically irreversible, and might also 

 be hereditary. 



But it was gradually realised that its employment brought in 

 its train other difficulties which were almost as great, so long, at 

 any rate, as one adhered to natural selection as the driving force 

 in evolution. This adhesion definitely handicapped the theory, 

 preventing it from giving its proper stimulus to biological progress. 

 Since people wished to combine it with natural selection, they 

 had to stipulate that mutations must be very small. It was very 

 hard to see how it could work with large mutations that might 

 effect such differences as distinguish the Monocotyledons from 

 the Dicotyledons, or even those that divide one family or genus 

 from another, and which might change the whole character of the 

 plant. If these were to be allowed, one could no longer imagine 

 progress by small, gradual, and progressive adaptation, and this, 

 determined in everj^ detail by natural selection, was still the 

 ruling principle invoked in evolution. If we remove direct 

 advantage from the list of factors that mav be immediatelv 

 operative in causing evolution to go on, it is evident that the 

 structural mutations that distinguish one form from another 

 need not, perhaps even cannot, proceed in gradual stages, unless 

 there be, as of course is by no means impossible, some at present 

 inscrutable law that guides them. But fossil evidence gives but 

 little support to this conception. Real intermediates are rare; 

 what are commonly called intermediates are usually things that 

 combine some of the characters of one with some of the other. If 

 one find a plant showing, to give an imaginary case, four of the 

 characters of Ranunculaceae to three of the Berberidaceae, it is 

 sure to give rise to discussion and dispute. 



