134 Evolution and Adaptation 



some cases been of high importance to an early progenitor ; 

 (3) the changed conditions of life may account for some of 

 the useless organs ; (4) reversion accounts for others; (5) the 

 complex laws of growth account for still others, such as 

 correlation, compensation of the pressure of one part on 

 another, etc. ; (6) the action of sexual selection is responsible 

 for many characters not to be explained by natural selection. 

 Admitting that there may be cases that can be accounted for 

 on one or the other of these six possibilities, yet there can be 

 no doubt that there are still a considerable number of specific 

 characters that cannot be explained in any of these ways. 

 I do not think that Darwin has by any means met this 

 objection, even if all these six possibilities be admitted as 

 generally valid. 



Amongst the '"miscellaneous objections" to his theory that 

 Darwin considers we may select the most important cases. 

 The following paragraph has been sometimes quoted by later 

 writers to show that Darwin saw, to a certain extent, the 

 insufficiency of fluctuating variations as a basis for selection. 

 What he calls here " spontaneous variability " refers to sudden 

 and extensive variations, or what we may call discontinuous 

 variations. " In the earlier editions of this work I under- 

 rated, as it now seems probable, the frequency and importance 

 of modifications due to spontaneous variability. But it is 

 impossible to attribute to this cause the innumerable struc- 

 tures which are so well adapted to the habits of life of each 

 species. I can no more believe in this, that the well-adapted 

 form of a race-horse or greyhound, which before the principle 

 of selection by man was well understood, excited so much 

 surprise in the minds of the older naturalists, can thus be 

 explained." 



Darwin appears to mean by the latter part of this state- 

 ment, that he cannot believe that such sudden and great 

 variations as have caused a peach tree to produce nectarines 

 can account for the wonderful adaptations of organisms ; but 



