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 

 conceivable that a naturalist, reflecting on the mutual 

 affinities 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. Nevertheless, 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 perfec- 

 tion of structure and coadaptation which justly excites 

 our admiration. Naturalists continually refer to ex- 

 ternal conditions, such as climate, food, &c, as the 

 only possible cause of variation. In one limited sense, 

 as we shall hereafter 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 



