274 STUDIES, SCIENTIFIC AND SOCIAL chap. 



extensive and very beautiful group of birds, though not 

 very numerous in species, ranging from the Himalayas to 

 Eastern Australia, but having one species isolated in 

 Western Africa. Dr. Sclater has himself mentioned a 

 few parallel cases, but there are others equally interesting, 

 a consideration of which may aid us in our attempted 

 explanation. We have first the gorilla and chimpanzee, 

 corresponding to the orangs of Borneo and Sumatra ; and 

 among the quadrumana the genus Cercocebus allied to 

 the Eastern macaques is wholly West African, while 

 Colobus, closely allied to the Asiatic Semnopithecus, is 

 found in Abyssinia as well as in West Africa. Among 

 birds we have Alethe, an African genus of babblers, 

 and Pholidornis, one of the Dicseidse, allied to Asiatic 

 forms ; while the genus of crested hornbills, Berenicornis, 

 has one species in Sumatra and the only other in West 

 Africa. The fruit-thrushes of the genus Criniger, so 

 abundant in Asia and Malaya, are also found in West and 

 South Africa, while the beautiful parroquets of the genus 

 Palaeornis inhabit West Africa and Abyssinia as well as 

 India and the larger Malay Islands. Among reptiles and 

 amphibia we have three families which follow the same 

 rule. Lizards of the family Acontiadae are confined to 

 the Moluccas, Ceylon, and West and South Africa ; toads 

 of the family Engystomidae have nearly the same range, 

 but are more widely spread in Asia ; while snakes of the 

 family Homalopsidse are abundant in tropical Asia and 

 America, and are even found in Europe, while in Africa 

 they are confined to the western districts. 



These numerous cases of the occurrence of what are 

 otherwise Eastern groups in West Africa, undoubtedly 

 suggest some correspondence of physical conditions which 

 renders this portion of the continent alone suitable to 

 them. The further question, how they got there at all, 

 is elucidated by what we know of the past history of 

 Africa and Europe. It is now generally admitted that, 

 before the Miocene period, Tropical and South Africa was 

 cut off from the great continent of the northern hemi- 

 sphere by a wide arm of the sea. It was then in fact an 

 island, or perhaps a group of large islands, and probably 



