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306 generation rooted to their native soil. The large numbers of persons who are found in this valley having the same surnames is a proof of the well-known fact that there is little or no emigration from the valley, and that there is an indefinite amount of intermarriage between the natives. It is conjectured that the continuance of these conditions through a long period has rendered the population more or less consanguineous, and that we see in crétinism an example, upon a large scale, of the evil effects of alliances of kindred.

This explanation commends itself by reason of its general applicability to crétinism. The disease is commonly found in valleys, on islands, or in other circumscribed areas, in which circulation is restricted, or the inhabitants are non-migratory; and it is rare on plains, where communications are free. It will at once be asked, "Why, then, are not the tributary valleys of the valley of Aosta full of crétins?" The answer is, that these lateral valleys are comparatively sterile, and are unable to support their population from their internal resources. Large numbers annually leave, and do not return,—some come back, having formed alliances elsewhere. There is a constant circulation and introduction of new blood. I am not aware that there are returns to show the extent to which this goes on, but the fact is notorious.