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Show 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. W e 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* requires a union in Miocene times between the oceans to account for the specific identity of certain corals ; Dr. Giintherf, 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 m y first and second requirements, 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. CATHARUS MEXICANUS, Bp. ; Scl. Cat. Am. B. p. 1. Calovevora • Cordillera del Chucu. * Quart. Journ. Geol. Soc. xix. 1863, p. 455. t Trans. Zool. Soc. vi. p. 397. |
