GEOGRAPHICAL DISTRIBUTION. 107 



A very narrow sea, if it be also very deep, 

 will sometimes form a well-marked, natural 

 .barrier between two totally distinct faunas and 

 floras. Thus, the channel which separates the 

 islands of Baly and Lombock, though only ten 

 miles wide, separates two of the primary 

 Eegions of Geographical Distribution, the 

 Indo-Malayan and Austro-Malayan, which are 

 not only far more sharply defined on their 

 boundaries than any other contiguous regions, 

 but are far less closely related, as regards the 

 character of their productions, than Africa 

 and India.* 



There is a close connection between the size 

 of animals, and the localities which they inhabit. 

 They are frequently smaller on islands than on 

 continents ; and when bred in confinement, are 

 apt to degenerate in size. This is supposed to be 

 due to want of room for sufficient variety in the 

 conditions of life, or to too close iriterbreeding.f 



The inhabitants of towns are usually smaller 

 arid weaker than residents in the country, 

 which is largely owing to the absence of 



* See Wallace's works. 



t Murphy, " Habit and Intelligence," vol. i. p. 189. 



