CHAPTER XXXIV 

 ADAPTATION 



" The free use of final causes to explain what seems obscure was temptingly 

 easy. . . . Hence the finalist was often the man who made a liberal use of the 

 ignava ratio, or lazy argument : when you failed to explain a thing by the ordinary 

 process of causality, you could ' explain ' it by reference to some purpose of nature 

 or of its Creator." 



Principal Galloway quoted by D'Arcy Thompson. 



If the environment exerts such an all-powerful effect on the 

 organism, can the organism alter itself according to the principle 

 of Le Chatelier so that it may live with the least possible expendi- 

 ture of energy ? That is, has the animal the power of adaptation ? 

 There is no doubt whatever as to the adaptation of growing bone 

 or growing tissue of any sort to the stresses and strains incident 

 upon it. Various organs are known to adapt themselves to meet 

 alterations in the conditions under which they work. 



When one comes to consider the organism as a whole, the 

 evidence for adaptation is not so conclusive. The Arctic fox and 

 the polar bear are not white because they have adapted themselves 

 to a white background, but because their coloured relatives have 

 paid the penalty consequent on their easy visibility against a white 

 backgroimd. It has been said that trypanosomes may be obtained 

 which are almost unaffected by treatment with arsenic. The pro- 

 cess for producing them is to give their host a high but non -lethal 

 dose of arsenic, infect another host with the survivors and so on. 

 This is clearly a case of the survival and propagation of the most 

 resistant strains. 



Animals which live in dark or semi-dark places have generally 

 defective eyesight. Is this due to atrophy from want of use or 

 might one not argue that the environment of the cave was the 

 fittest for the blind or semi-blind animal ? Not only would they 

 be at a manifest disadvantage in the struggle for existence outside, 

 but they have a distinct advantage in the cave over any seeing 

 animal that may stray in. 



To be brief, one nuist consider that, as anything but a rapid 

 response to the distribution of forces in the environment is in- 

 compatible with life, the animal capable of adapting itself to 

 circumstances will live and probably propagate. Man, because of 



462 



