Here Steiner seems to be saying that American Indians are visibly ossifying under the influence of the strange Saturnian emanations that his supposedly infallible clairvoyance had revealed to him:
"Finally, what we may describe as the abnormal Spirits of Form who have their centre in Saturn, act upon the glandular system, but in a roundabout way through all the other systems. Therefore in all that we must describe as the Saturn-race, in everything to which we must attribute the Saturn-character, we must look for something which draws together and embraces that which leads again to the evening twilight of humanity, whose development brings humanity in a certain way to a real conclusion, to a dying away. The expression of this action on the glandular system is seen in the American-Indian race. From that action comes its mortality, its disappearance. The Saturn influence acts through all the other systems finally upon the glandular system. It separates out the hardest parts of man, and we may therefore say that this dying-out consists in a sort of ossification, and this may also clearly be seen in the outer form.
If you look at the pictures of the old American Indians, the process above described is palpable in the decline of this race. In a race such as this, everything which existed in the Saturn-evolution is now present in them, and that in a special manner; it has withdrawn into itself and left man alone with his hard bone system, and brought him into decline." [p. 78-79]
He continues, in one of the most mawkish passages I've ever read:
"One feels something of this truly occult activity, if one observes how, even in the nineteenth century, a representative of these old Indians speaks of how in him there dwells what formerly was great and mighty for man, but which could not possibly go along with further evolution. There is in existence a description of a beautiful scene, in which a leader of these Indians who are dying out, confronts a European invader.
Imagine what is felt in the heart when two such men confront each other, men who came across from Europe, and men who in the earliest ages, when the races were divided, went over to the West. The Indians then took over with them to the West all that was great in the Atlantean culture. What was the greatest thing of all to the Indian? It was that he was still able dimly to sense something of the ancient greatness and majesty of a period which existed in the old Atlantean epoch, in which the division of the races had hardly begun, in which men could look up to the Sun and perceive the Spirits of Form penetrating through a sea of mist. Through an ocean of mist the Atlantean gazed up at that which to him was not divided into six or seven, but which acted together. This co-operative activity of the seven Spirits of Form was called by the Atlanteans the Great Spirit who revealed himself to man in ancient Atlantis. The Atlantean had not taken into himself all that the Venus, Mercury, Mars and Jupiter Spirits brought about in the East, through which were developed all the civilisations which reached their zenith in Europe in the middle of the nineteenth century. In all this the son of the brown race did not participate. He clung firmly to the Great Spirit of the primeval past. That which the others had done, those who in a primeval past had also received the Great Spirit, passed before his eyes when a paper was laid before him on which were many little signs, letters, of which he understood nothing. All that was foreign to him, but in his soul he still had the Great Spirit. His speech has been preserved to us; it is worthy of note because it points to what we have explained, and it runs somewhat as follows: 'There, in the ground upon which walk the conquerors of our country, the bones of my brothers are buried. Why are the feet of our conquerors allowed to walk over the graves of my brothers? Because they are in possession of that which makes the white man great. The brown man is made great by something else; he is made great by the Great Spirit, Who speaks to him in the sighing of the wind, in the rustling of the forest, in the surging of the waves, in the gurgling of the spring, in thunder and lightning! That is the Spirit Who to us speaks truth. Oh, the Great Spirit speaks truth! Your Spirits,whom you have here on paper, and who express what to you is great, they do not speak truth.' Thus spoke the Indian Chief, from his point of view. The brown man belongs to the Great Spirit; the pale man belongs to the spirits who, in black shapes, as little dwarflike beings - he meant the letters - hop about on the paper and who do not speak the truth. That is a world-historic dialogue, which was carried on between the conquerors and the last of the great Chiefs of the brown men. Here we see what belongs to Saturn and his activity, and what originates on the earth from his co-operation with other Spirits, at such a moment as this, when two different directions meet." [p.79-80]
It's simple. Indians had to die out: but don't worry, the conquerors were simply following their cosmic destiny.