

434 PALEONTOLOGY. 



nations of land and sea brought about in the seons of geological 

 time. Yet this reasoning is applicable only to land-animals ; 

 for it is scarcely conceivable that such operations can have 

 affected sea-fishes. 



There are characters in land animals rendering them more 

 obnoxious to extirpating influences, which may explain why 

 so many of the larger species of particular groups have become 

 extinct, whilst smaller species of equal antiquity have sur- 

 vived. In proportion to its bulk is the difficulty of the con- 

 test which, as a living organism, the individual of such 

 species has to maintain against the surrounding agencies 

 that are ever tending to dissolve the vital bond, and subjugate 

 the living matter to the ordinary chemical and physical 

 forces.* Any changes, therefore, in such external agencies as 



* The influence of the contest for existence, amidst the changes of the cir- 

 cumstances to which an animal has "been adapted, on the extinction of species, 

 was first propounded by the author in his fourth memoir on Dinornis, 1850 

 (Trans, of the Zool. Society, vol. iv., p. 15). The same principle has since been 

 evoked to explain not only the extinction but the origin of species. 



Mr. Wallace,* assumes that a variety may arise in a wild species, adapting it to 

 changes in surrounding conditions, under which it has abetter chance of existence 

 than the type-form from which it deviated, and of which it would take the place. 



Mr. Charles Darwin had, previously to Mr. Wallace, conceived the same 

 application of this principle, which he illustrates in his work " On the origin of 

 Species," by many ingenious suppositions, such as the following: — "To give 

 an imaginary example from changes in progress on an island — let the organi- 

 zation of a canine animal which preyed chiefly on rabbits, but sometimes on 

 hares, become slightly plastic ; let these same changes cause the number of 

 rabbits very slowly to decrease, and the number of hai'es to increase ; the effect 

 of this would be that the fox or dog would be driven to try to catch more hares; his 

 organization, however, being slightly plastic, those individuals with the lightest 

 forms, longest limbs, and best eyesight, let the difference be ever so small, 

 would be slightly favoured, and would tend to live longer, and to survive during 

 that time of the year when food was scarcest ; they would also rear more young, 

 which would tend to inherit these slight peculiarities. The less fleet ones 

 would be rigidly destroyed. I can see no more reason to doubt that these causes 

 in a thousand generations would produce a marked effect, and adapt the form 

 of the fox or dog to the catching of hares instead of rabbits, than that grey- 

 hounds can be improved by selection and careful breeding." f Yet this con- 



* Proceedings of the Linnsean Society, August 1858, p. 57. 

 t Proceedings of the Linnsean Society, August 185S, p. 49. 



