72 Illustrations of the Action of Chap. :r. 



would have only a slightly better chance of surviving and breeding; 

 and this chance would go on decreasing in the succeeding genera- 

 tions. The justice of these remarks cannot, I think, be disputed. 

 If, for instance, a bird of some kind could procure its food more- 

 easily by having its beak curved, and if one were born with its beak 

 strongly curved, and which consequently nourished, nevertheless- 

 there would be a very poor chance of this one individual perpetuat- 

 ing its kind to the exclusion of the common form ; but there can 

 hardly be a doubt, judging by what we see taking place under 

 domestication, 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 bein^ 

 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 would 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 selection. Or only a third, fifth, or 

 tenth part of the individuals may have been thus affecteM, 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 Una 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, t sh n h t recur; but . t ^ ^ ^ 



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



birds wtlTl T ^ ab ° Utj W6 See this even *» ^ory 

 birds, wh ch almost always return to the same spot. Consequently 

 eacn newly-formed variety would ^nwaiu*^ /* \ ^ un ^L u * iibl J 

 in i,o +t, q Lr» i ^y wouia generally be at first local, as seems 



to be the common rule with varieties in a e* a f„ ^ ± ^ *, 



srimiiowu, ,*w~«fl i • i- •-. ytU4ew «»i»a state ot nature; so that 

 sum krly modified individuals would sooa exist in a small body 

 together, and would often breed toa-stW i*\l 

 fiit«np*flft,i «„ u i **i e 1-7 . TO £ § ™*« « the new variety were 

 successiul m its battle lor life, it wauld al«wi« j * ^ 



i " WOIUCI slowly spread from a central 



