BIOLOGY IN ITS WIDER ASPECTS 1315 



struggle is not in itself the source of progress; it must have varia- 

 tions, or differences of endowment, to work on. 



{b) Moreover the struggle for existence does not necessarily make 

 for evolution. In many cases it thins without sifting, and that does 

 not make for racial change. Out of 533 caterpillars of the large 

 garden white butterfly collected by Prof. Poulton, 422 died because 

 Ichneumon flies had laid their eggs inside them. This was serious 

 thinning, four out of five, but so far as we know those caterpillars 

 that escaped being victimised were no better than those that 

 perished, so that there was no sifting. The indiscriminate elimination 

 involved in thinning turnips with a hoe benefits the surviving 

 individuals, but it does not improve the race of turnips. The only 

 result of the struggle for existence that is of direct evolutionary 

 importance is discriminate elimination, where the presence or 

 absence of a particular character determines survival; or, what 

 comes to the same thing in the long run, determines relative success 

 in producing and rearing progeny. For it must be understood that 

 the process of selection is often very slow, and even gentle in its 

 operations. In some cases the struggle for existence forces creatures, 

 without much or any organic change, to enter a new habitat or 

 adopt a new habit. This again is not necessarily of much evolutionary 

 importance, unless it is associated with a weeding out of the non- 

 plastic, or brings secondary organic changes in its wake. 



(c) We must also notice the obstinate confusion of thought, that 

 selection in the struggle for existence must, automatically as it were, 

 result in the survival of something desirable. What it results in is 

 the survival (immediate or distant) of the relatively more fit to the 

 conditions of life. It may work towards degeneration as well as 

 towards progress ; as is well illustrated by that evasion of the struggle 

 for existence called parasitism — the door to which is always open. 

 The liver fluke is "fit" as well as the sheep, and the tapeworm is as 

 well adapted to its inglorious lot as the lark at heaven's gate. The 

 survivors of discriminate elimination are the relatively fittest to 

 given conditions — which might be hell. 



(d) But there is an even deeper misunderstanding. In spite of 

 many protests, beginning with Darwin's own, the idea of the struggle 

 for existence has often been expressed in a narrow and wooden way. 

 It is a fact of life much bigger and subtler than the words suggest, 

 and we do well to repeat Darwin's proviso that the phrase was to be 

 used "in a large and metaphorical sense, including dependence of 

 one being on another, and including (which is more important) 

 not only the life of the individual, but success in leaving progeny". 

 This full concept is far from being easy; Darwin himself confessed 

 that he found it difficult to keep it always clearly in his mind. The 

 phrase is a shorthand formula, summing up a vast variety of strife 

 and endeavour, of thrust and parry, of action and reaction. 



