56 



about in the seons of geological time. Yet this reasoning is appli- 

 cable 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 ob- 

 noxious 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 survived. In pro- 

 portion to its bulk is the difficulty of the contest which the animal 

 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, there- 

 fore, in such external agencies as a species may have been origi- 

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

 degree proportionate to the size which may characterise the spe- 

 cies. If a dry season be gradually prolonged, the large mammal 

 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 nourishment ; if new 

 enemies be introduced, the large and conspicuous animal will fall a 

 prey while the smaller kinds conceal themselves and escape. Small 

 quadrupeds, moreover, are more prolific than large ones. Those 

 of the bulk of the mastodons, megatheria, glyptodons, and dipro- 

 todons, are uniparous, The actual presence, therefore, of small spe- 

 cies of animals in countries where larger species of the same na- 

 tural families formerly existed, is not the consequence of degenera- 

 tion 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 to which the 

 larger species have succumbed. 



That species should become extinct appears, from the abundant 

 evidence of the fact of extinction, to be a law of their existence; 

 whether, however, it be inherent in their own nature, or be rela- 

 tive and dependent on inevitable changes in the conditions and 

 theatre of their existence, is the main subject for consideration. 

 But, admitting extinction as a natural law which has operated 

 from the beginning of life on this planet, it might be expected 

 that some evidence of it should occur in our own time, or within 

 the historical period. Reference has been made to several in- 

 stances of the extirpation of species, certainly, probably, or pos- 

 sibly, due to the direct agency of man ; but this cause avails not in 



