XIV DISTRIBUTION 627 



tropics. Finally, the dispersal of many fixed or shore-hauntino' 

 animals is ensured by their free-swimming larvje. ° 



Importance of the various Groups of Animals in Zoo- 

 Geography.— In close dependence on the means of dispersal we 

 have the fact that the various groups of animals are of very 

 unequal value in the study of distribution. The greater the 

 facilities for the transport of any species across a given barrier, 

 the less significance will attach to its occurrence on both sides of the 

 barrier. Conversely, when a species, having few or no facilities for 

 dispersal, is found on opposite sides of an important barrier, the 

 natural conclusion is either that the barrier is of comparatively 

 recent formation, and that the two areas separated by it were once, 

 so to speak, in zoological continuity, or that the species in question 

 is a very ancient one, and was widely dispersed at a time when the 

 arrangement of the land-surface was very different from what it is 

 at the present day. For instance, the occurrence of strong-flying 

 Birds, such as Gulls and Cormorants, in widely separated countries, 

 is a fact of no significance in determining the mutual relationships 

 of the faunae of those countries. But the occurrence of the same 

 species of Fresh-water Crayfish— to which the narrowest arm of 

 the sea is an insuperable barrier— in Great Britain and the 

 European Continent, is explained only by the fact — of which 

 there is independent evidence— that the English Channel is of 

 recent formation. And when we find the various species of 

 Peripatus dotted over the earth's surface in an apparently casual 

 manner, we are forced to the conclusion that this genus must 

 formerly have been very widely and continuously distributed and 

 subsequently exterminated over the greater part of its range ; 

 since it is hardly possible to conceive of either the adult or the 

 young of this creature, living in rotten wood in the recesses of the 

 forest, having been transported between Australia and New 

 Zealand, or between Africa and the West Indies. 



Speaking generally, then, it may be said that discontinuity in 

 the distribution of a species or other group is evidence of its 

 antiquity. In addition to Peripatus, the Dipnoi and the Tapirs 

 may be mentioned as examples. 



It will be seen that terrestrial and fresh-water animals are of 

 more importance, from the point of view of zoo-geography, than 

 marine forms. Among the inhabitants of the sea, littoral species 

 are of greater significance than pelagic or abyssal. Amongst 

 land animals, those which are unable to swim, and those which 

 cannot survive immersion in salt water, are of more importance 

 than strong swimmers, or than such forms as are able to live for a pro- 

 longed period on drift-wood, or in mud attached to the feet of 

 Birds. 



In connection with what has been said above about there being 

 no special significance to be attached to the distribution of certain 



