On the other hand, these variations are not greater than can be adduced in various members of the white or black race. In spite of them all, there is a wonderful family likeness among tribes of American origin. No observer well acquainted with the type would err in taking it for another. Darwin says that the Fuegians so closely resemble the Botocudos that they seem members of the same tribe. I have seen Arawacks from Guiana who in the northwest would have passed for Sioux. In spite of the total dissimilarity of climate and other physical surroundings, the tribes of the tropics differ no more from those near the Arctic circle than they do among themselves. This is a striking lesson how independent of environment are the essential characteristics of a race, and it is a sweeping refutation of those theories which make such characteristics dependent upon external agencies. A still more remarkable fact has been demonstrated by Professor J. Kollmann of Bâle: to wit, that the essential physical identity of the American race is as extended in time as it is in space. This accurate student has analyzed the cranioscopic formulas of the most ancient American skulls, those from the alleged tertiary deposits of the Pampas, those from the caverns of Lagoa Santa in Brazil, that obtained from Rock Bluff, Illinois, the celebrated Calaveras skull from California, and one from Pontemelo in Buenos Ayres of geologic antiquity. His results are most interesting. These very ancient remains prove that in all important craniologic indicia the earliest Americans, those who were contemporaries of the fossil horse and other long since extinct quadrupeds, possessed the same racial character as the natives of the present day, with similar skulls and a like physiognomy.[26] We reach therefore the momentous conclusion that the American race throughout the whole continent, and from its earliest appearance in time, is and has been _one_, as distinct in type as any other race, and from its isolation probably the purest of all in its racial traits. This is a fact of the first order in establishing its prehistoric chronology. _Geologic._ I have left the geologic data to the last, as it is these which carry us with reasonable safety to the remotest periods. No one who examines the evidence will now deny that man lived in both North and South America during and after the glacial epochs, and that he was the contemporary of many species of animals now extinct. As you are aware, the attempt has several times been made to fix the date for the final retrocession of the glaciers of North America. The estimates have varied from about 12,000 years ago up to 50,000, with a majority in favor of about 35,000 years.