CHAPTER XXXIII 



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, not 

 having their invisibility, have paid the penalty. It has been said 

 that trypanosomes may be obtained which are almost unaffected 

 by treatment with arsenic. The process 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. 



360 



