CHAP. XXll. 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 organized productions, compared with marine and 

 lower productions, b}' the more complex relations of the 

 higher beings to their organic and inorganic conditions of 

 life."* 



If we suppose the mammalia to be more sen«itive than arc 

 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 

 ujjon 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 varj^, upon the whole, at a more rapid 

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

 supposed that tliey 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 organization take place, Avhen new conditions 

 arise, appears to be well exemplified by the absence even 

 of small warm-blooded quadru2:)eds in islands far from con- 

 tinents, however well such islands may be fitted by their 

 dimensions to support them. 



Mr. Darwin has pointed to this absence of mammalia as 

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



* Origin of Species, od ed., p. 340. 



