INTRODUCTION 



they could lay more eggs — the egg industry is founded upon the 

 inexhaustible gullibility of the domestic fowl — and could, of 

 course, lay fewer. Having regard to all the exigencies of giving 

 birth to and rearing eggs and young, the size of its clutch is just 

 about that which gives each species its greatest likelihood of 

 self-perpetuation . 



A second misconception may be aptly called the Zenonian, 

 because of a certain family likeness to the argument which 

 purports to show that Achilles can never overtake the tortoise, 

 nor an arrow reach its target. Any substantial adaptation, it 

 is argued, can only be achieved by the adding up, over very 

 many generations, of single all but infinitesimal adaptative 

 changes which, being of inappreciable advantage to their 

 owners, offer nothing for selection to get to grips upon. Luckily 

 selection does not abide by human judgements of its efficacy; 

 it can be shown that even so slight a selective advantage as that 

 which allows one thousand and one of its possessors to per- 

 petuate themselves for every thousand that lack it, must 

 eventually prevail. 



Philosophers have sometimes beguiled themselves with a 

 third argument allegedly discreditable to Darwinism. Selection 

 (the argument runs) can select only from what is available and 

 "'given'', i.e. can select only from within the compass of existing 

 variation and diversity; by selection, all men or all deer could 

 be caused to become as tall or swift as the tallest or swiftest 

 now among them, but not still taller or swifter still. The error 

 here is to equate all variation with overt variation, to suppose 

 that all genetic goods are in the shop window. The genetical 

 mechanism is such that there are deep resources of hidden 

 variation, of possible animals only awaiting the occasion to 

 become real. Even if mutation were to take a holiday, as Death 

 did in Casella"'s play, evolution would certainly not come to a 

 standstill; how far it could go we obviously cannot say. 



I think I have given reasons enough to justify a certain pre- 



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