XIII EVOLUTION AND DISTRIBUTION OF ANIMALS 273 



example of a dying-out group, of which two allied species 

 remain isolated in remote countries favourable to their 

 existence ; and further, that the supposition of their 

 common ancestors having once occupied the intervening 

 region, so far from being unwarranted, is supported by 

 the analogy of several other groups of birds in the same 

 area. 



Dr. Sclater's next two cases may be sufficiently ex- 

 plained by the application of the general considerations 

 and examples already adduced. We have two allied 

 species of Oxyrhamphus isolated in South-East Brazil and 

 Central America — countries, it may be remarked, about 

 equally removed from the equator and enjoying very 

 similar climates; while in the cuckoos of the genus 

 Neomorpha we have a similar phenomenon in nearly the 

 same two areas, with the addition of three species in the 

 intervening districts which are not closely allied to the 

 two others. 



We have here really only the same class of facts as 

 occur plentifully in the Palsearctic region, in which the 

 species of the eastern and western extremes are often 

 alike, while those that intervene are more diverse. We 

 can, without much difficulty, refer this latter peculiarity 

 to diversity or similarity of climate and phj^sical condi- 

 tions, while in the Neotropical region it is more probable 

 that a diversity of organic conditions may have been the 

 agent at work. There is some reason to believe that the 

 great plateau of Guiana long formed an island, and that 

 this isolation led to the development of several peculiar 

 forms, which have in some cases spread into Upper Ama- 

 zonia. A range of plateaux and hills, on the other hand, 

 connects Brazil with the Andes, and has thus kept up a 

 greater zoological continuity with Central America than 

 the intervening area of Guiana has been able to do. 



Dr. Sclater's third problem, that of Pitta angolensis, is 

 very interesting, and will affi^rd us an opportunity of dis- 

 cussing some of the most curious phenomena of distribu- 

 tion, and of bringing forward some considerations which I 

 believe will go far towards the removal of most of the 

 difficulties they present. The case is that of a rather 



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