INTRODUCTION. 19 



see, this may bo true ; but it is preposterous to attribute 

 to mere exteriuil conditions, the structure, for instance, of 

 the woodpecker, with its feet, tail, beak, and tonjTue, so ad- 

 mirably adapted to catch insects under the bark of trees. In 

 the case of the mistletoe, which draws its nourishment from 

 certain trees, M'hich has seeds that must be transported by cer- 

 tain birds, and which has flowers with separate sexes absolutely 

 requiring the ag^ency of certain insects to brin<^ pollen from one 

 flower to the other, it is equally preposterous to account for 

 the structure of this parasite, with its relations to several dis- 

 tinct organic beings, by the efl'ects 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 coramencem(mt of my observations it seemed to me prob- 

 able that a carcfyl study of domesticated animals and of culti- 

 vated plants would oiler 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 variation under domesti- 

 cation, aflbrded the best and safest clew. I may venture to 

 express my conviction of the high value of such studies, al- 

 though they have been very commonly neglected by natural- 

 ists. 



From these considerations, I shall devote the first chapter 

 of this Aljstract 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 jiower 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 cir- 

 cumstances are most favorable to variation. In the next chap- 

 ter the Struggle for Existence among all organic beings 

 througliout the world, which inevitably follows from the high 

 geometrical ratio of their increase, Avill be treated of. This is 

 the doctrine of Malthus, applied to the whole animal and vege- 

 table kingdoms. As many more individuals of each species 

 are born than can possil)ly survive ; and as, consequently, there 

 is a frequently-recurring struggle for existence, it follows that 

 any being, if it vary however slightly in any manner profitable 



