INTRODUCTION. 3 



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 nourishment from certain trees, which has seeds that 

 must be transported by certain birds, and which has flowers 

 with separate sexes absolutely requiring the agency of cer- 

 tain insects to bring pollen from one flower to the other, it 

 is equally preposterous 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 

 provable 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 

 clue. I may venture to express my conviction of the high 

 value of such studies, although they have been very com- 

 monly neglected by naturalists. 



From these considerations, I shall devote the first chap- 

 ter of this abstract to variation under domestication. We 

 shall thus see that a large amount of hereditary modifica- 

 tion is at least possible; and, what is equally or more im- 

 portant, 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, how- 

 ever, be enabled to discuss what circumstances are most 

 favorable to variation. In the next chapter the strug- 

 gle for existence among all organic beings throughout 

 the world, which inevitably follows from the high geomet- 

 rical ratio of their increase, will be considered, This is 



