1870.) MR, O. SALVIN ON THE BIRDS OF VERAGUA. 179 
portions of the Isthmus, and some others, we find that northern forms 
found in Central America are specifically identical with northern 
species, and that their presence is due in a great measure to migration 
during the winter season. As regards numbers, we find a gradual 
diminution as we proceed away from North America. These 
migrants, however, are everywhere present, some few passing still 
further south into the equatorial provinces of the southern continent. 
Costa Rica and Veragua, with Panama, possess these characteristics 
of the Central-American fauna in the highest degree. It is here we 
find the greatest number of South-American genera represented ; but 
the species are to a considerable extent not the same as the continental 
species. 
We find, too, a considerable number of northern migrants, most 
of which are specifically identical with northern birds. 
In endeavouring to account for the facts as we find them, by 
changes in past times in the physical features of the Isthmus, we seem 
to require :—1st. A union between Costa Rica, Veragua, and Panama 
with the southern continent, when those united lands possessed in 
common a much larger number of species specifically the same than 
at present. During this time the oceans may have been united north 
of Costa Rica. 2nd. The long duration of Costa Rica and Veragua 
as a “continental ”’ island, during which time the union of the two 
oceans has been of greater extent. This period must be long enough 
to have established specific differences much as we now find them. 
3rd. The emergence of the whole Isthmus in its present form. 
These requirements seem to fall in fairly with what has been 
demanded in other branches of natural science. Dr. Duncan* re- 
quires a union in Miocene times between the oceans to account for 
the specific identity of certain corals; Dr. Giinther+, too, requires a 
union between the oceans to account for the specific identity of 30 
per cent. of the fish now found on both sides of the Isthmus. 
The union here demanded will suit my first and second require- 
ments, I only regulate the amount; and as for the period when it 
took place, the fixing it to Miocene times would seem to answer to 
the requirements of the birds. 
That all the peculiar features of so varied a fauna can be accounted 
for by this theory I do not pretend to say. The changes in the 
physical features of the Isthmus indicated by the numerous minor 
modifications of existing species, belong to the most recent events in 
geological history. To account for the greater differences observable 
we must go deeper into the abyss of geological time, where light at 
present is barely perceptible. 
Catharus griseiceps. 
Chitra ; Calovevora; Calobre. 
1. CarHARUS MEXICANUS, Bp. ; Scl. Cat. Am. B. p. 1. 
Calovevora ; Cordillera del Chucu. 
* Quart. Journ. Geol. Soc. xix. 1863, p. 455, 
+ Trans. Zool. Soe. vi. p. 397. 
