CH. II] REGIONS OF MR SCLATER. 73 



other groups and they have been for the most part 

 accepted. Mr Wallace more than anyone else has written 

 much in their support. 



It has however been insisted upon by many that these 

 regions do not fit in with the facts of distribution of other 

 groups. That their applicability to the Passerines and to 

 those groups which they do suit is due to the fact that 

 these groups are modern and that there has in all proba- 

 bility not been much change in the relative distribution 

 of land and sea since the groups in question came into 

 existence. These regions however are more particularly 

 unsuitable to older groups, which retain, so to speak, the 

 impression of earlier conditions of land and sea. So much 

 so that the agreement or non-agreement of a particular 

 group with the regions instituted by Mr Sclater are in 

 some degree a test of its antiquity. Even in the more 

 modern groups the resemblance is not always striking. 

 Nor could we really expect that it would be ; for a close 

 resemblance would imply a similar place of origin, an 

 identical series of migrations and backward migrations, 

 and a susceptibility to precisely the same barriers and 

 hindrances ; these assumptions are evidently not to be 

 thought of as well founded. So complicated are the 

 conditions which govern the restrictions to migration and 

 the facilitation of migration that it would be impossible 

 to conceive of there being in any one case close corre- 

 spondence with another case. While therefore we cannot 

 expect to find a series of cut and dried regions which shall 

 express the known facts of distribution of all terrestrial 

 groups, it is of some use to have a convenient system 



