42 PHYSICAL FEATURES ETC. 
isolated in the mountains of the higher 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 recede 
from North America. These migrants 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. If we endeavour 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 as at present, during which time the oceans may have been 
united north of Costa Rica. 2nd. The long duration of Costa Rica and Veragua 
as a ‘continental’ island, when 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. 38rd. 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 requires a union in Miocene times 
between the oceans to account for the specific identity of certain corals. The union 
here demanded will suit my first and second requirements, I only regulate the amount ; 
and as for the period when it took place, 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 is at present barely perceptible.” 
In his first paper on this subject (P.Z.S. 1867, pp. 129-161), based upon less 
extensive material, Salvin stated that there was a closer affinity between the birds of 
Veragua and those of Costa Rica than between those of Veragua and of the Isthmus 
of Panama, but this proved not to be the case when Arcé’s later collections were 
examined. He then remarked that it was evident that Costa Rica and Panama had 
for a long period occupied the position of one or more islands between the two 
continents at a time when the two oceans were united by two or more channels; and 
that an obvious division separating Costa Rica, Veragua, and Panama from the 
southern continent was a line drawn from the Atlantic Bay of San Blas to the mouth 
of the Bayano on the Pacific. 
