A thousand years may pass over two nations undoubtedly of the same origin; and which were, at the beginning of those thousand years, of the same complexion, form, and language.
At the end of those thousand years there shall be a difference. With one the language shall have changed rapidly, the physical structure slowly.
With the other the physical conformation shall have been modified by a quick succession of external influences, whilst the language shall have stayed as it was.
With an assumed or proved original identity on each side, the difference in the rate of action on the part of the different influences, is the key to all discrepancies between the two tests. The language may remain in statu quo, whilst the hair, complexion, and bones change; or the hair, complexion, and osteology may remain in statu quo, whilst the language changes.
Apparently this leaves matters in an unsatisfactory condition; in a way which allows the ethnologist any amount of assumption he chooses. Apparently it does so; but it does so in appearance only. In reality we have ways and means of determining which of the two changes is the likelier.
We know what modifies form. Change of latitude, climate, sea-level, conditions of subsistence, conditions of clothing, &c., do this; all (or nearly all) such changes being physical.
We know, too, (though in a less degree) what modifies language. New wants gratified by objects with new names, new ideas requiring new terms, increased intercourse between man and man, tribe and tribe, nation and nation, &c. do this; all (or nearly all) such changes being of a moral nature.
Hence in some cases we can ascertain upon which of the two elements of our classification, the physical or the moral, the greatest amount of influences has been at work.
It is necessary to remark upon these points because it is only physically that the tribes of the present division are nearest akin to those of the previous ones. Had similarity of language been the test, a different and a more distant class of nations would have formed the subject of the present section.