22 INTRODUCTION 



authorities alone. I can here give only the general conclusions at 

 which I have arrived, with a few facts in illustration, but which, 

 I hope, in most cases will suffice. No one can feel more sensible 

 than I do of the necessity of hereafter publishing in detail all the 

 facts, with references, on which my conclusions have been 

 grounded; and I hope in a future work to do this. For I am 

 well aware that scarcely a single point is discussed in this volume 

 on which facts cannot be adduced, often apparently leading to 

 conclusions directly opposite to 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 satis- 

 faction 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 con- 

 clusion that species have 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 perfection of 

 structure and coadaptation which justly excites our admiration. 

 Naturalists continually refer to external conditions, such as 

 climate, food, etc., 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 nourish- 

 ment from certain trees, which has seeds that must be trans- 

 ported by certain birds, and which has flowers with separate 

 sexes absolutely requiring the agency of certain insects to bring 

 pollen from one flower to the other, it is equally preposterous to 



