Geographical Distribution 139 



by climatic differences and so forth. He raised the 

 questions of modes of dispersal and of barriers to dis- 

 persal, of similarities due to common descent, and of the 

 modifying results produced by isolation. He gave, in 

 fact, a theory of the " creations" which Mr. Sclater 

 had shewn to be a probable assumption. It was in the 

 nature of things that Huxley should make a contribu- 

 tion to a set of problems so novel and of so much 

 importance to zoology. In 1868, in the course of a 

 memoir on the anatomy of the gallinaceous birds and 

 their allies, he made a useful attempt, nearly the first 

 of its kind, to correlate anatomical facts with geo- 

 graphical distribution. Having shewn the diverging 

 lines of anatomical structure that existed in the group 

 of creatures he had been considering, he went on to 

 shew that there was a definite relation between the 

 varieties of structure and the different positions on the 

 surface of the globe occupied at the present time by 

 the creatures in question. He made, in fact, the geo- 

 graphical position a necessary part of the whole idea of 

 a species or of a group, and so introduced a conception 

 which has become a permanent part of zoological 

 science. 



With regard to the number and limits of the zoologi- 

 cal regions into which the world may be divided, Hux- 

 ley raised a number of problems which have not yet 

 reached a full solution. Mr. Sclater had divided the 

 world into six great regions : the Nearctic, including 

 the continent of North America, with an overlap into 

 what is called South America by geographers ; the 

 Palaearctic, comprising Europe and the greater part of 

 Asia; the Oriental, containing certain southern portions 

 of Asia, such as India south of the Himalayas and many 

 of the adjacent islands; the Ethiopian, including Africa, 



