INTRODUCTION. 3 



those at. which I have arrived. A fair result can be 

 obtained only by fully stating and balancing the facts 

 and arguments on both sides of each question; and this 

 is here impossible. 



I much regret that want of space prevents my having 

 the satisfaction of acknowledging the generous assistance 

 which I have received from very many naturalists, some 

 of them personally unknown to me. I cannot, however, 

 let this opportunity pass without expressing my deep 

 obligations to Dr. Hooker, who, for the last fifteen years, 

 has aided me in every possible way by his large stores 

 of knowledge and his excellent judgment. 



In considering the Origin of Species, it is quite con- 

 ceivable that a naturalist, reflecting on the mutual afiini- 

 ties of organic beings, on their embryological relations, 

 their geographical distribution, geological succession, 

 and other such facts, might come to the conclusion 

 that species had not been independently created, but 

 had descended, like varieties, from other species. Never- 

 theless, such a conclusion, even if well founded, would 

 be unsatisfactory, until it could be shown how the 

 innumerable species inhabiting this world have been 

 modified, so as to acquire that perfection of structure 

 and coadaptation which justly excites our admiration. 

 Naturalists continually refer to external conditions, 

 such as climate, food, &c., as the only possible cause 

 of variation.^ In one limited sense, as we shall here- 

 after see, this may be true; but it is preposterous to 

 attribute to mere external conditions, the structure, for 

 instance, of the woodpecker, with its feet, tail, beak, and 

 tongue, so admirably adapted to catch insects under 

 the bark of trees. In the case of the mistletoe, which 

 draws its nourishment from certain trees, which has 



