1 6 In vcrtebrata. 



are white or colourless. Many large caves, like those 

 of Kentucky, Adelsberg c., have thus peculiar blind 

 inhabitants. 



Sometimes the presence of one animal prevents 

 the diffusion of others ; thus in Africa the tzetze fly 

 renders whole tracts uninhabitable by oxen and horses, 

 which are destroyed by its poisonous bites. 



The fauna of a limited area of a continent usually 

 exceeds that of an island of equal size in its number 

 of specific forms; and the fauna of an island lying near 

 a continent generally resembles that of its neighbour- 

 ing continent. Oceanic islands or those isolated by 

 very deep straits have often remarkable faunae of their 

 own, e.g. the Galapagos and New Zealand. 



Tropical species are, as a rule, more limited in 

 range than are those of temperate climates, and simpler 

 animals are usually more widely distributed than are 

 the more complex. 



Fresh- water inhabitants are the fewest specifically, 

 and as a rule are simpler in organisation than allied 

 forms inhabiting the sea. The fourth sub-kingdom 

 has no fresh- water representatives ; the second has only 

 three, and the third only six species living in this 

 medium ; while the others are not very numerously 

 represented in fresh water. 



The sea is the home of nine-tenths of the inver- 

 tebrates (if we exclude insects), and there are also 

 definite ranges of extension to be noticed in the 

 cases of marine species. The conditions limiting 

 specific life in the sea are depth, currents, and 

 temperature. 



Terrestrial animals are the most specialised, and 



