88 SECTIONAL ADDRESSES 



All that natural selection can ensure is survival. It does not ensure 

 progress, or maximum advantage, or any other ideal state of affairs. A 

 type may survive by deceiving its enemies with a fraudulent imitation of 

 a nauseous form just as well as by some improvement in digestion or 

 reproduction, by degenerate and destructive parasitism as much as by 

 increased intelligence. 



Then we must invoke natural selection whenever an adaptive structure 

 involves a number of separate steps for its origin. A one-character, single- 

 step adaptation might clearly be the result of mutation. But when two 

 or more steps are necessary, it becomes inconceivable that they shall have 

 originated simultaneously. The first mutation must have been spread 

 through the population by selection before the second could be combined 

 with it, the combination of the first two in turn selected before the third 

 could be added, and so on. Most adaptations clearly involve many 

 separate characters, and when we can study their actual evolution with 

 the aid of fossils, we find that it is steadily progressive over tens of millions 

 of years, and must therefore have involved a large number of steps. 

 The improbability is therefore enormous that they can have arisen without 

 the operation of some agency which can gradually accumulate and combine 

 a number of contributory changes : and natural selection is the only such 

 agency that we know. 



R. A. Fisher has aptly said that natural selection is a mechanism for 

 generating a high degree of improbability. This is in a sense a paradox, 

 but it expresses epigrammatically the important fact that natural selection 

 is all the time achieving its results by giving probability to combinations 

 which would otherwise be in the highest degree improbable. 



This important principle clearly removes all force from the ' argument 

 from improbability ' used by many anti-Darwinians, such as Bergson. 

 It helps us also to detect another fallacy. T. H. Morgan, followed 

 by Hogben, has asserted that natural selection merely preserves certain 

 among the hosts of recombinations : in the absence of natural selection, 

 in addition to the known forms of life a vast assemblage of other types 

 would exist which have been destroyed by selection. 



Actually this is on a par with saying that we could expect the walls 

 of a room to collapse on occasion owing to all the molecules of gas inside 

 the room moving simultaneously in one direction. Both are of course 

 only improbabilities — -but they are improbabilities of such a fantastically 

 high order as to be in fact entirely ruled out. Each single existing species 

 is the product of a long series of selected mutations ; to produce these 

 adapted types by chance recombination in the absence of selection would 

 require a total assemblage that would fill the universe and overrun 

 astronomical time. 



This is perhaps the place to discuss pre-adaptation. According to 

 this view, variations occur which would be adaptive in some new environ- 

 ment or way of life, and their possessors then find their way into that 

 environment or take up that way of life. However, what we have pre- 

 viously said makes it clear that this can only apply to the early stages of an 

 elaborate adaptation, not to its whole history. 



A mutation such as that discovered by Banta for altered temperature- 

 resistance in a Cladoceran may be described as potentially pre-adaptive ; 



