GEOGRAPHICAL DISTRIBUTION OF VERTEBRATE ANIMALS. 405 



graphs and local lists of species take note chiefly of political 

 (rather than natural) areas. 



Certain special maps, which may be called " Key Maps," 

 would be also necessary, and these are noticed hereafter. 



4. Mapping out the Distribution of Species. — The exact dis- 

 tribution of any species having been more or less precisely 

 ascertained from published or other sources, and copies of the 

 map of the world being ready, the next matter obviously is to 

 portray the area inhabited by the species in question on one 

 of the maps, in order that its distribution, the world over, and 

 the more important facts connected with, or influencing, its 

 distribution, may be shown cartographically at a glance, as 

 already explained. 



Birds, in consequence of their power of migration, will 

 obviously require a much more detailed scheme of mapping, in 

 order adequately to represent the geographical distribution of the 

 various species, than any other division of vertebrate animals 

 will require, except perhaps the Fishes. In no other class of the 

 vertebrate animals do we meet with anything corresponding with 

 migration among birds, in the same sense and to the same 

 extent. In the case of Birds, it is necessary to portray both 

 their summer and winter distribution, as well as the lines of 

 migration in both spring and autumn (often different), and the 

 spots outside the ordinary range of the species at which indi- 

 viduals have occurred as accidental stragglers — a point which 

 (though comparatively trivial when a single or a few species only 

 are under consideration) becomes of some importance in a com- 

 prehensive scheme like the present, as indicating former or 

 at present unsuspected lines of migration. 



The Geographical Distribution of birds being, therefore, more 

 complicated and more difficult to represent pictorially upon a 

 map than the distribution of any other class of vertebrate 

 animals, the following scheme has been drawn up specially with 

 reference to the class Aves ; but it will be found, I believe, that 

 there is no fact in connection with the geographical distribution 

 of either the Mammals or Keptiles (and probably none connected 

 with that of the Fishes) which is not capable of being readily 

 indicated by a partial or simplified use of this scheme. 



