xiv DISTRIBUTION 591 



fixed or shore-haunting animals is ensured by their free-swimming 

 larvae. 



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 signi- 

 ficance will attach to its occcurrence 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 con- 

 clusion is either that the barrier is of comparatively recent forma- 

 tion, 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 arrange- 

 ment 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 crea- 

 ture, 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 men- 

 tioned 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 about the distribution of certain strong- 



