402 THE ZOOLOGIST. 



(b). Its summer and winter ranges throughout the world (if 



migratory) ; 

 (c). Its relative abundance in different parts of its area; 

 (d). Its lines of migration (if any) ; 



(e). The additional area (if any) over which any species, now 

 partly or wholly extinct, can be traced within historic 

 times ; 

 (/). The additional area (if any) over which it has been 



naturalized by human agency ; and 

 (g). Many other points of interest, such as isolated occurrences,* 

 erratic movements,! areas of hybridization,! &c, &c. 

 The interest and value attaching to such a work, if properly 

 carried out, must be at once apparent to any one acquainted 

 with the subject. Geographical Distribution is a key to many 

 very interesting problems in natural science, and a scheme such 

 as this, if perfected, should throw much light on many vexed 

 questions as to the origin of species, and on the value of the 

 distinctions which naturalists have drawn between what we 

 are accustomed to regard as "good" or "bad" species. It 

 should show, to some extent at least, the causes which have 

 separated, or are separating, species, whether those causes of 

 separation are deep seas, mountain ranges, variations of tem- 

 perature, or other physical facts ; while similar remarks apply 

 to sub-species (those incipient and partially -differentiated species 

 to which "trinomial" names are now applied). The careful 

 mapping-out of the distribution of these latter forms should 

 throw some light on the physical causes which are separating 



* I refer here more especially to such isolated and exceptional occurrences 

 as those discussed by Mr. J. J. Dalgleish in his " List of Occurrences of North 

 American Birds in Europe" (Bull. Nutt. Orn. Club, vol. v. 1880, pp. 65 — 74, 

 141—150, and 210—221). 



f By this term, I mean such movements as those occasionally performed 

 by Syrrhaptes paradoxus and Lemmus norvegicus, which are not regular or 

 seasonal, and are therefore quite distinct from migration proper. 



I For instance, of Corvus comix and C. corone and of Lanius excubitor 

 and L. major in Western Siberia ; also of Colaptes auratus and C. mexicanus, 

 and many other species, in North America. In most cases, areas of hybrid- 

 ization will be found at the dividing line between the breeding grounds of a 

 species and one of its own sub-species ; but this is by no means always the 

 case, as may be seen on reference to Monsieur Suchetet's ' Les Oiseaux 

 hybrides rencontres a l'etat sauvage.' 





