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, hut 6 of them are peculiar. The Australian region 
has 8, the Palaearetic 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 deserta 
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 Megalaemidse and Trogo niche over the tropics 
of America, Africa, and Asia. In the absence of palaeonto- 
logical evidence as to the former history of The Megalsemidse, 
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, Meropidae 
and Coraciidae are equally characteristic of the Oriental and 
