3 1 ILLUSTEATIOKS OF TUE ACTION OF Cuap. IV. 



and breeding- ; and this chance would go on decreasing' in the 

 succeeding generations. The justice of these remarks cannot, 

 I think, be disputed. If, for instance, a bird of some kind 

 couhl procure its food more easily by having its beak curved, 

 and if one Avere born with its beak strongly carved, and 

 wliich consequently flourished, nevertheless there would be 

 a very poor chance of this one individual perpetuating its kind 

 to the exclusion of the common form ; but there can hardly be 

 a doubt, judging by what we see taking place imder domesti- 

 cation, that this result Avould follow from the preservation dur- 

 ing many generations of a large number of indiWduals with 

 more or less curved beaks, and from the destruction of a still 

 larger numlier Avith the straightest beaks. 



It should not, however, be overlooked that certain varia- 

 tions, which no one would rank as mere individual differences, 

 frequently recur, owing to a similar organization being simi- 

 larly acted on — of which fact numerous instances could be 

 given with our domestic productions. In such cases, if a vary- 

 ing individual did not actually transmit to its offspring its 

 newly-acquired character, it would imdoubtedly transmit, as 

 long as the existing conditions remained the same, a still 

 stronger tendency to vary in the same manner. The condi- 

 tions might indeed act in so energetic and definite a manner as 

 to lead to the same modification in all the individuals of the 

 species without the aid of selection. But we may suppose 

 that the conditions sufficed to effect only a third, or fourth, or 

 tenth part of the individuals ; and several such cases could be 

 given ; for instance, it has been estimated by Graba that in the 

 Faroe Islands about one-fiftli of the guillemots, Avhich all breed 

 together, consist of a well-marked variety ; and this Avas for- 

 meily ranked as a distinct species under the name of Uria 

 lacrymans. Now, in such cases, if the variation were of a ben- 

 eficial nature, the original form would soon be sup})lantcd by 

 the modified form, through the survival of the fittest. 



With reference to the eftects of intercrossing and of com- 

 petition, it should be borne in mind that most animals antl plants 

 keep to their proper homes, and do not needlessly Avander 

 aljout ; Ave see this CA'cn Avith migratory birds, Avhich almost 

 always return to the same district. Consequently each newly- 

 formed variety Avould generally be at lirst local, as seems to 

 1)0 the common rule Avith A'arieties in a state of nature ; so that 

 siinilarlA'-modified individuals Avould soon exist in a small l)ody 

 together, and would often breed together. If the v.vw variety 



