Chap. IV.] NATURAL SELECTION. 113 



owing to a similar organisation being similarly acted on 

 — of which fact numerous instances could be given with 

 our domestic productions. In such eases, if the varying 

 individual did not actually transmit to its offspring its 

 newly-acquired character, it would undoubtedly trans- 

 mit 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 tend- 

 ency to vary in the same manner has often been so 

 strong that all the individuals of the same species have 

 been similarly 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 laerymans. In eases of this kind, if the variation 

 were of a beneficial nature, the original form would soon 

 be supplanted by the modified form, through the sur- 

 vival of the fittest. 



To the effects of intercrossing in eliminating varia- 

 tions 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. Consequently 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 



