Chap. IV.] NATUEAL 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 cases, if the varying 

 individual did not actually transmit to its offspring its 

 newly-acquired character, it w T ould undoubtedly 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 tendency 

 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 selec- 

 tion. Or only a third, fifth, or tenth part of the indi- 

 viduals 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 o± 

 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. 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 district, competing 



