xxxiv PREFACE. 



try affecting the sustenance or -well-being of its indigenous animals, 

 may lead not only to their modification but to their destruction. I 

 have, in another work, pointed out the characters in the animals 

 themselves calculated to render them most obnoxious to such ex- 

 tirpating influences ; and have applied the remarks to the expla- 

 nation of so many of the larger species of particular groups of 

 animals having become extinct, whilst smaller species of equal 

 antiquity have remained. 



i In proportion to its bulk is the difficulty of the contest which, 

 as a living organised whole, 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 a species may have been originally 

 adapted to exist in, will militate against that existence in a degree 

 proportionate, perhaps in a geometrical ratio, to the bulk of the 

 J species. If a dry season be gradually prolonged, the large mam- 

 mal will suffer from the drought sooner than the small one ; 

 if such alteration of climate affect the quantity of vegetable food, 

 the bulky Herbivore will first feel the effects of stinted nourish- 

 ment ; if new enemies are introduced, the large and conspicuous 

 quadruped or bird will fall a prey, whilst the smaller species con- 

 ceal themselves and escape. Smaller animals are usually, also, 

 more prolific than larger ones.' l 



The actual presence, therefore, of small species of animals in 

 countries where larger species of the same natural families for- 

 merly existed, is not the consequence of any gradual diminution 

 of the size of such species, but is the result of circumstances, 

 which may be illustrated by the fable of the ' Oak and the Reed ; ' 

 the smaller and feebler animals have bent and accommodated 

 themselves to changes which have destroyed the larger species. 

 They have fared better in the ' battle of life.' 



Accepting this explanation of the extirpation of species as true, 



1 On the Genus Dinomis (Part iv.), Zool. Trans., vol. iv. p. 15 (February 1850). 



