Rheas
 
Darwin's Rhea (Rhea darwinii)

The ornithologist John Gould identified Darwin's South American "ostrich" or "petisse" as a new species [middle above, & below], and named it Rhea darwinii after its discoverer. Its range is adjacent to that of the common rhea, Rhea americana [left]. Darwin wrote in his notebook in March 1837, "The same relation that common ostrich bears to Petisse, extinct guanaco to recent: in former case position, in latter time." That is, geographical relationships mirror analogous relationships in time: there is evidence of transmutation of species. Darwin's biographer Janet Browne (1995) says "This moment more than any other in Darwin's life deserves to be called a turning point."

Rhea
          darwinii

2. RHEA DARWINII. Gould.

PLATE XLVII.

Gould, in Proceedings of Zoological Soc. 1837, p. 35.

R. pallide fusca, plumâ singulâ distinctâ semilunari notâ candidâ terminatâ; capite collo, femoribusque pallidioribus: rostri culmine augusti, ad apicem latiore, frontes plumis parvis setosis anticè directis et supra nares extensis; tarsi lateribus in dimidiam partem plumis parvis mollibus tectis; tarso 2/3 antice posticeque toto, squamis reticulatis tecto.

Long. tot. 52 unc.; alæ, 30; tarsi, 11; rostri, 2.

The whole of the plumage light brown, each feather with a decided crescent-shaped mark of pure white at the extremity; head, neck, and thighs lighter; base of the neck blackish; culmen of the bill narrow, becoming a little broader towards apex; front with small bristly feathers, pointing forwards and reaching over the nostrils. Tarsus with small downy feathers on sides, extending half way downwards; upper two-thirds of front of tarsus, and whole hinder side, with reticulated scales.

Habitat, Eastern Patagonia (Lat. 40° S. to 54° S.)

This species, which Mr. Gould, in briefly characterizing it at a meeting of the Zoological Society, has done me the honour of calling after my name, differs in many respects from the Rhea Americana. It is smaller, and the general tinge of the plumage is a light brown in place of grey; each feather being conspicuously tipped with white. The bill is considerably smaller, and especially less broad at its base; the culmen is less than half as wide, and becomes slightly broader towards the apex, whereas in the R. Americana it becomes slightly narrower; the extremity, however, of both the upper and the lower mandible, is more tumid in the latter, than in the R. Darwinii.


Plates and Gould text from "Zoology of the Voyage of the HMS Beagle" (1839); Text material © 2020 by Steven M. Carr