10 CONTACTS WITH DARWINISM I [ch. ii 



properly so-called. Some new theoretical background is required, 

 other than natural selection, which has proved a very broken 

 reed upon which to lean. 



Those who have tried to make evolution work upon Darwinian 

 lines, i.e. in the "upward " direction from minute variety through 

 variety and species, and so on, have met with continually 

 increasing difficulties, with some of which we now propose to deal. 



For the variations that were ultimately to form the basis of 

 new species, Darwin relied principally upon the "infinitesimal" 

 or continuous variation that was well kno\Mi always to be going 

 on in every possible character. Thus, supposing one measured 

 the length of 500 leaves from similar plants of the same species, 

 one might find the average to be 25 mm. The greatest single 

 number would probably be found to show this length, but there 

 would be almost, if not quite, as many measuring 24 or 26 mm., 

 somewhat fewer for 23 and 27 mm., and falling away more and 

 more quickly, but at about the same rate on either side. Investi- 

 gation gradually showed that there were definite limits to this 

 kind of variation. It follows the ordinary curve of frequency 

 distribution. If one cross two individuals both having a very 

 high degree of the character, the average of their offspring does 

 not retain that high level, but falls back, or regresses. The high 

 level can only be maintained by strenuous selection in each 

 generation. Further, it is also found by experience that one 

 cannot, by means of selection, pass a certain maximum. This 

 kind of variation, in other words, is not fully hereditary nor is it 

 irreversible, like the differences that characterise species, and 

 cannot be indefinitely added up without some external aid. The 

 experiences of sugar beet and other breeding show this well 

 enough; never can one go beyond a certain point unless, by 

 hybridisation or in other ways, one introduces new factors. In 

 the struggle for existence, mere chance has much too large a share 

 in determining the victors to allow even the maximum to be 

 reached. Thus, on this ground alone, this type of variation was 

 disqualified as forming an essential part of the evolutionary 

 mechanism. 



But this is not the only difficulty that arises in trying to use 

 this kind of variation, which is always linear, or up-and-down. 

 A leaf may vary infinitesimally in length, or in breadth, in the 

 depth of its incisions, or in the degree of number and length of its 

 hairs, but it does not vary except in sudden steps in such a 

 direction as that from alternate to opposite, from simple to com- 



