INTRODUCTION. 3 



this may be true ; but it is preposterous to attribute to mere 

 external conditions, the structure, for instance, of the wood- 

 pecker, 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 cer- 

 tain trees, which has seeds that must be transported 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 preposter- 

 ous to account for the structure of this parasite, with its 

 relations to several distinct organic beings, by the effects of 

 external conditions, or of habit, or of the volition of the 

 plant itself. 



It is, therefore, of the highest importance to gain a clear 

 insight into the means of modification and coadaptation. 

 At the commencement of my observations it seemed to me 

 probable that a careful study of domesticated animals and 

 of cultivated plants would offer the best chance of making 

 out this obscure problem. Nor have I been disappointed; 

 in this and in all other perplexing cases I have invariably 

 found that our knowledge, imperfect though it be, of vari- 

 ation under domestication, afforded the best and safest clew. 

 I may venture to express my conviction of the high value of 

 such' studies, although they have been very commonly neg- 

 lected by naturalists. 



From these considerations, I shall devote the first chapter 

 of this abstract to variation under domestication. We shall 

 thus see that a large amount of hereditary modification is at 

 least possible ; and, what is equally or more important, we 

 shall see how great is the power of man in accumulating by 

 his selection successive slight variations. I will then pass 

 on to the variability of species in a state of nature ; but I 

 shall, unfortunately, be compelled to treat this subject far 

 too briefly, as it can be treated properly only by giving long 

 catalogues of facts. We shall, however, be enabled to discuss 

 what circumstances are most favorable to variation. In the 

 next chapter the struggle for existence among all organic 

 beings throughout the world, which inevitably follows from 

 the high geometrical ratio of their increase, will be considered. 

 This is the doctrine of Malthus, applied to the whole animal 

 and vegetable kingdoms. As many more individuals of each 

 species are born than can possibly survive ; and as, conse- 

 quently, there is a frequently recurring struggle for exist- 

 ence, it follows that any being, if it vary however slightly 



