The Mechanism of Adaptation 329 



the cells of the body; and how explain the union of homolo- 

 gous maternal and paternal chromosomes in synapsis and 

 their unique method of separation in the reduction division, 

 upon which processes the phenomena of Mendelian inheri- 

 tance depend? How is it possible to explain the adaptive 

 mechanisms of the egg and sperm and of the processes of 

 fertilization? How can we explain the teleological character 

 of embryonic development, in which the end is apparently 

 in view from the beginning? How is it possible to account 

 for the adaptive tropisms, reactions, and instincts of ani- 

 mals, the complicated but delicately adjusted relationship 

 between different individuals and species, their ingenious 

 means of defense and offense and the surprising efficiency 

 of the living machine? Finally, is it possible to find any 

 natural and causal explanation of the adaptations of indi- 

 viduals to conditions which neither they nor their ancestors 

 have ever before experienced? 



The list of such fitnesses is well-nigh endless, and the 

 question of their origin forms one of the most striking and 

 fundamental problems of biology. It may be necessary for 

 the biologist to disregard this problem for the present be- 

 cause he cannot deal with it, but he should never forget 

 that it is a real problem and challenges scientific explanation. 

 This subject is undoubtedly a dangerous one for the scien- 

 tist, full of pitfalls for the unwary and with many alluring 

 calls to metaphysical speculation; but it lies in the back- 

 ground of every biological problem. As Professor W. K. 

 Brooks taught, "Life is response to the order of nature," 

 and it is the element of useful and apparently purposive 

 response which more than anything else distinguishes the 

 living from the lifeless, and separates the methods and re- 

 sults of biology from those of chemistry and physics. 



Innumerable attempts have been made by philosophers 



