I inherited an old Bible (1757), obviously well used in the 19th century. It contained several odd pieces of paper which have been fascinating from a genealogical viewpoint.

The following was scribbled in pencil on a scrap of paper. It's hard to read, but I've given it my best shot:

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"It has been ascertained by late observations, that not only the inhabitants of Unalashka but several tribes of South America, indicate by the osteological characters of the head, a passage (?) from the American to the Mongol race.

When we shall have more completely studied the brown men of Africa, and that swarm of nations who inhabit the interior and North West of Asia, and who are vaguely described by systematic travellers under the names of Tartars and Tschoudes (?), - the Caucasian, Mongol, American, Malay and Negro races, will appear less insulated, and we shall acknowledge, in this great family of the human race, one single organic type, modified by circumstances which perhaps will ever remain unknown."

Introduction to Humboldt's Researches concerning the Institutions and Monuments of the Ancient Inhabitants of South America etc.

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Can anyone supply or confirm particularly the two emboldened words, and possily even correct any other errors I may have made?

I'd guess 'passage' is right, in its sense of 'link/transition/migration'. Presumably, the point is that early Americans probably did descend from Asiatic people who crossed into Alaska via the land-bridge that existed at one time. (Or is the suggestion here that the opposite occurred?)
As for 'tschoudes', the closest the OED offers is 'tschoadar', a variant on 'chobdar', which is Urdu for 'beadle/usher'. One quote refers to a chobdar as 'a person of the meanest station', so maybe it was used at one time just to imply 'the masses' of Asia.

Many thanks, MQ. You were up very early this morning!

The original definitely looks like 'Tschoudes', but as with anything copied it may have been written down wrong. Given that it was probably written by a woman in the mid-19C, it could well have been a mishearing of 'tschoadar'. I suppose that's as close as I'm going to get without locating Humboldt's work in print.

Incidentally, I have seen 'the masses' misprinted as 'them asses'!