43G PAL/EONTOLOGY. 



measure, may be due to constitutional changes accumulating 

 by slow degrees in the long course of generations, is possible ; 

 but all hitherto observed causes of extirpation point either 

 to continuous slowly operating geological changes, or to no 

 greater sudden cause than the, so to speak, spectral appear- 

 ance of mankind on a limited tract of land not before inhabited. 

 It is most probable, therefore, that the extinction of species, 

 prior to man's presence or existence, has been due to ordinary 

 causes — ordinary in the sense of agreement with the laws of 

 never-ending mutation of the geographical and climatal con- 

 ditions on the earth's surface. The species, and individuals 

 of species least adapted to bear such influences, and incapable 

 of modifying their organization in agreement therewith, have 

 perished. Extinction, therefore, on this hypothesis, implies 

 the want of self-adjusting power in the individuals of the 

 species subject thereto. 



But admitting extinction as a natural law, which has 

 operated from the beginning of life under specific forms of 

 plants and animals, it might be expected that some evidence 

 of it should occur in our own time, or within the historical 

 period. Eeference has been made to several instances of the 

 extirpation of species, certainly, probably, or possibly, due to 

 the direct agency of man. The hook-billed parrot (Nestor 

 productus) of Philip's Island, west of New Zealand, is, perhaps, 

 the latest instance of this kind. But this cause avails not in 

 the question of the extinction of species at periods prior to 

 any evidence of human existence ; it does not help us in the 

 explanation of the majority of extinctions, as of the races of 

 aquatic Invertebrata and Vertebrata which have successively 

 passed away. 



The Great Auk (A lea impennis, L.) seems to be rapidly 

 verging to extinction, if it be not exterminated ; and that not 

 wholly, as in the case of the dodo and dinornis, by the hand 

 of man. Some of the geological changes affecting circum- 



