xiv DISTRIBUTION 607 



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 faunse 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 prolonged period on driftwood, 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 

 strong-flying Birds, it must be remarked that this is by no means 

 true of migratory Birds. Many British Birds, such as the Swallow, 

 Cuckoo, Swift, &c., spend the summer in England, the winter in 

 South Europe or Africa. One of the New Zealand Cuckoos winters 

 in Australia, the others in Fiji or some other Pacific islands. Birds 

 capable of such feats of flight might, one would think, soon 

 overspread the globe ; yet, as a matter of fact, each species is 

 found to keep strictly to its own definite line of migration, even 

 across 1,000-1,500 miles of sea. 



Having now indicated the general character of the facts and 

 problems connected with the subject of zoo-geography, we may 



p p 2 



