'CHAP. xxn. ABSENCE OF MAMMALIA IN ISLANDS. 443 



Mr. Darwin remarks, 'We can, perhaps, understand the 

 apparently quicker rate of change in terrestrial, and in more 

 highly organised productions, compared with marine and 

 lower productions, by the more complex relations of the 

 higher beings to their organic and inorganic conditions of 

 life.* 



If we suppose the mammalia to be more sensitive than are 

 the inferior classes of the vertebrata, to every fluctuation in 

 the surrounding conditions, whether of the animate or inani 

 mate world, it would follow that they would oftener be called 

 upon to adapt themselves, by variation, to new conditions, or 

 if unable to do so, to give place to other types, This would 

 give rise to more frequent extinction of varieties, species, and 

 genera, whereby the surviving types would be better limited, 

 and the average duration of the same unaltered specific types 

 would be lessened. 



Absence of Mammalia in Islands considered in Reference 

 to Transmutation. 



But if mammalia vary, upon the whole, at a more rapid 

 rate than animals lower in the scale of being, it must not be 

 supposed that they can alter their habits and structures 

 readily, or that they are convertible in short periods into new 

 species. The extreme slowness with which such changes of 

 habits and organisation take place, when new conditions 

 arise, appears to be well exemplified by the absence even of 

 small warm-blooded quadrupeds in islands far from continents, 

 however well such islands may be fitted by their dimensions 

 to support them. 



Mr. Darwin has pointed to this absence of mammalia as 

 favouring his views, observing that bats, which are the only 



* Origin of Species, 3rd ed. p. 340. 



