ACTION OF NATURAL SELECTION 105 



that this result would follow from the preservation during 

 many generations of a large number of individuals with more 

 or less strongly curved beaks, and from the destruction of a 

 still larger number with the straightest beaks. 



It should not, however, be overlooked that certain rather 

 strongly marked variations, which no one would rank as mere 

 individual differences, frequently recur owing to a similar 

 organisation being similarly acted on — of which fact numer- 

 ous instances could be given with our domestic productions. 

 In such cases, if the varying individual did not actually trans- 

 mit to its offspring its newly-acquired character, it would un- 

 doubtedly transmit to them, as long as the existing conditions 

 remained the same, a still stronger tendency to vary in the 

 same manner. There can also be little doubt that the ten- 

 dency to vary in the same manner has often been so strong 

 that all the individuals of the same species have been simi- 

 larly modified without the aid of any form of selection. Or 

 only a third, fifth, or tenth part of the individuals may have 

 been thus affected, of which fact several instances could be 

 given. Thus Graba estimates that about one-fifth of the 

 guillemots in the Faroe Islands consist of a variety so well 

 marked, that it was formerly ranked as a distinct species 

 under the name of Uria lacrymans. In cases of this kind, if 

 the variation were of a beneficial nature, the original form 

 would soon be supplanted by the modified form, through the 

 survival of the fittest. 



To the effects of intercrossing in eliminating variations of all 

 kinds, I shall have to recur ; but it may be here remarked that 

 most animals and plants keep to their proper homes, and do 

 not needlessly wander about ; we see this even with migratory 

 birds, which almost always return to the same spot. Conse- 

 quently each newly-formed variety would generally be at 

 first local, as seems to be the common rule with varieties in a 

 state of nature; so that similarly modified individuals would 

 soon exist in a small body together, and would often breed 

 together. If the new variety were successful in its battle for 

 life, it would slowly spread from a central district, competing 

 with and conquering the unchanged individuals on the mar- 

 gins of an ever-increasing circle. 



It may be worth while to give another and more complex 



