CHAP. XVIII.] BIRDS. 323 



while no less than 15 out of the 25 families are exclusively 

 tropical, none are confined to, or have their chief development 

 in, the temperate regions. They are best represented in the 

 Ethiopian region, which possesses 17 families, 4 of which are 

 peculiar to it ; while the Oriental region has only 14 families, 

 none of which are peculiar. The Neotropical region has also 

 14 families, but 6 of them are peculiar. The Australian region 

 has 8, the Palsearctic 9 and the Nearctic 6 families, but none 

 of these are peculiar. We may see a reason for the great 

 specialization of this tropical assemblage of birds in the Ethio- 

 pian and Neotropical regions, in the fact of the large extent of 

 land on both sides of the Equator which these two regions alone 

 possess, and their extreme isolation either by sea or deserts 

 from other regions, — an isolation which we know was in both 

 cases much greater in early Tertiary times. It is, perhaps, for 

 a similar reason that we here find hardly any trace of the 

 connection between Australia and South America which other 

 groups exhibit; for that connection has most probably been 

 effected by a former communication between the temperate 

 southern extremities of those two continents. The most 

 interesting and suggestive fact, is that presented by the dis- 

 tribution of the Megalsemidee and Trogonidse over the tropics 

 of America, Africa, and Asia. In the absence of palseonto- 

 logical evidence as to the former history of the Megalsemidee, 

 we are unable to say positively, whether it owes its present 

 distribution to a former closer union between these continents 

 in intertropical latitudes, or to a much greater northern range 

 of the group at the period when a luxuriant sub-tropical vege- 

 tation extended far toward the Arctic regions ; but the dis- 

 covery of Trogon in the Miocene deposits of the South of 

 France renders it almost certain that the latter is the true 

 explanation in the case of both these families. 



The Neotropical region, owing to its enormous family of 

 humming-birds, is by far the richest in Picarise, possessing 

 nearly half the total number of species, and a still larger pro- 

 portion of genera. Three families, the Bucerotidee, Meropidse 

 and Coraciidae are equally characteristic of the Oriental and 



