TRANSLA TOR \S RREFA CE 



XXI 



of its body that it lived in trees. But as a matter of fact 

 it does live entirely in trees. 



Variation and survival are the explanation of adaptations, 

 but the struggle for existence does not merely ^ive rise to 

 natural selection — it is the cause of the variations ^iiicli 

 survive. 



Selection, whether natural or artificial, is perfectly analo- 

 gous to the process of denudation in geology. It explains 

 the extinction of innumerable forms and the consequent 

 gaps and intervals which separate species, families, ordere, 

 etc., just as denudation explains the want of continuity in 

 the stratified rocks. But geologists have never been blind 

 enough to suppose that the evolution of the structure of a 

 given rock was due to denudation ; they have always believed 

 that the structure of each rock was due to the effects of tlie 

 forces which have acted upon it since its formation, and 

 they have devoted their energies to tracing by observation 

 and experiment the effects of the various forces. It lias 

 been the distinction of biologists to maintain the paradox 

 that matter in a certain condition, namely, in the form of 

 living organisms, is not subject to the action of physical 

 forces. 



But the following argument, which I recently attempted 

 to publish in a letter to the Editor of Nature, seems to me to 

 be stronger than any I have yet used. The letter containing 

 it never appeared in the journal, which professes to represent 

 natural science in this country. Its suppression was the 

 more surprising because the controversial correspondence to 

 which it belonged arose originally from a letter of mine, 

 which was published in the same journal, on the explanation 

 of the eyes of flat-fishes given in Wallace's Darwinian. 



