but then it came to those of us which had sought knowledge of the man-things in the land of longer summers that the man-things had often used a certain kind of low-tree to drive us away from their things-to-eat , for the low-to-the-ground clouds which come from that particular low-tree make it hard for us to breathe , and over the passage of many periods of light and darkness , many of our kind have died when they could no longer breathe .