Studies on the Structure of Feather Keratin: II. A β-Helix Model for the Structure of Feather Keratin

The assumption that the proline residues in feather keratin, which comprise 12 per cent of the total, are periodically located along the polypeptide chain is shown to lead to an essentially unique structure for this fibrous protein. The structure is based on a β-helix; i.e., an extended chain which coils slowly to form a helix of relatively large pitch. Such helices tend to aggregate by hydrogen bonding to form cylindrical units, which in turn can aggregate further into cable-like structures. This model has been tested with respect to its predictions concerning the x-ray diffraction pattern, infrared spectrum, mechanical properties, and chemical behavior of feather keratin. Preliminary results indicate that it is better capable of accounting for the data than previously proposed structures.     

XIII.—On the Age of Some Races of Birds.

For the purpose of this discussion "races" are assumed to be of genetical significance. With this caveat the conclusions are offered for what they are worth.
On different lines of argument it appears that two races, Galerida cristata nigricans and Columba livia dakhlÆ have developed in the last 10,000 years.
Galerida cristata brachyura and Prinia gracilis natronensis have probably differentiated in less than 5000 years.
The fact that no Passerine races have developed in Mesopotamia may indicate that the lower figure, namely 5000 years, is not far from the minimum required for the development of an ornithological race.