CiiAP. X. EXTINCTION. 301 



existing; elephant. A lii;;lily-capablc judge, Dr. Falconer, be- 

 lieves that it is chielly insects which, from incessantly harass- 

 ing and weakening the elephant in India, check its increase ; 

 and this was Bruce's conclusion with respect to the African 

 elephant in Abj'ssinia. It is certain that insects and blood- 

 sucking bats determine the existence of the larger naturalizec 

 quadrupeds in several parts of South America. 



We sec in many cases in the more recent tertiary forma- 

 tions, that rarity precedes extinction ; and we know that this 

 has been the progress of events with those animals which have 

 been exterminated, eillier locally or wholly, through man's 

 agency. I may repeat what I published in 1815, uamel}', that 

 to admit that sjiecies generally become rare before they be- 

 come extinct — to feel no surprise at the rarity of a species, 

 and yet to marvel greatly when the species ceases to exist, is 

 much the same as to admit that sickness in the individual is 

 t)ie forerunner of death — to feel no surprise at sickness, but, 

 when the sick man dies, to wonder and to suspect that he died 

 by some deed of violence. 



The theory of natural selection is grounded on the belief 

 that each new variety, and ultimately each new species, is pro- 

 duced and maintained by having some advantage over those 

 with which it comes into competition ; and the consequent 

 oxtinctioii of the less-favored forms almost inevitably follows. 

 It is the same with our domestic productions ; when a new 

 and slightly-improved variety has been raised, it at first sup- 

 plants the less improved varieties in the same neighborhood; 

 Avhen much improved, it is transported far and near, like our 

 short-horn cattle, and takes the place of other breeds in other 

 countries. Thus the a})pcarance of new forms, and the disap- 

 pearance of old forms, both those naturally and those artifi- 

 cially produced, are bound together. In nourishing groups, 

 the number of new specific f(jrms which have been produced 

 within a given time, has at some periods probably been greater 

 than the number of the old specific forms which have been ex- 

 terminated; but we know tliat species have not gone on indefi- 

 nitely increasing, at least during the later geological epochs ; 

 so that, looking to later times, we may believe that the pro- 

 duction of new forms has caused the extinction of about the 

 same number of old forms. 



llie competition will generally be most severe, as formerly 

 explained and illustrated by examples, between the forms 

 which are most like each other in all respects. Hence the 



