CHAP. IV.] XATURAL SELECTION. 67 



vidual perpetuating 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 indi- 

 viduals 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 organisa- 

 tion 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 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 selec- 

 tion. 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 guille- 

 mots 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. 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 varietj* 

 were successful in its battle for life, it would slowly spread from 

 a central district, competing with and conquering the unchanged 

 individuals on the margins of an ever-increasing circle. 



It may be worth while to give another and more complex 

 illustration of the action of natural selection. Certain plants 

 excrete sweet juice, apparently for the sake of eliminating some- 

 thing injurious from the sap : this is effected, for instance, by 

 glands at the base of the stipules in some Leguminosae, and at the 

 backs of the leaves of the common laurel. This juice, though 

 small in quantity, is greedily sought by insects ; but their visits 

 do not in any way benefit the plant. Now, let us suppose that 



