animals of the arctic region were driven hundreds of miles south of the great chain of lakes on the northern boundary of the United States. Many of the animals of preglacial times were unable to stand the strain of the ever-changing climatic conditions and have become extinct, but their fossil remains are left to tell the story to the present and future ages. Much of the history of those times is a sealed book, but the persevering energy of the glacialist and archæologist is gradually turning the leaves of this old book and revealing new chapters of the wonderful story of the ice. As the ice receded the arctic zone again traveled northward, and many animals, plants, and trees that had survived the vicissitudes of the ice age, traveled back with it. Some of them, however, became acclimated and by adapting themselves to the new conditions remained behind to live and grow with the aborigines of preglacial times. Some of the plants and flowers that grew in profusion immediately under the edge of the great ice sheet were unable to live under the new conditions of increased warmth--that came with the retrograde movement of the ice--and either had to follow closely the receding ice or escape to higher altitudes, where they found a congenial clime. Thus it is that we have arctic plants and flowers above the timber line and near the snow line of our high mountains. In proof of this theory it has been found that these arctic plants do not exist upon high mountains, such as the Peak of Teneriffe, where they have been isolated from the glaciated region. The Peak of Teneriffe is situated on one of the Canary Islands, surrounded by water, so that there was no possible chance for the arctic plants to seek refuge on these isolated elevations, such as the continental mountains furnish. Thus it will be seen that the progression and recession of the ice have not only formed great lakes, changed river beds, and covered a million square miles of area with glacial drift averaging fifty feet in depth, making many waterfalls and giving variety to the surface of the earth, besides producing the finest agricultural region in the world, but have also given variety to our forests and plants wherever this ice sheet has extended. CHAPTER XXIX. DRAINAGE BEFORE THE ICE AGE. We have already said that during the ice age river-beds were changed, valleys were filled up, new lakes were made, and waterfalls created. Great as were the changes made by the carrying power of moving ice, still greater were those made in preglacial times; not, however, from the action of moving ice, but from running water. Erosion caused by running water has, probably, during the life of the world, transported more material from place to place, from mountain to valley, and from valley to ocean, than any other agency; chiefly for the reason that it has been so much longer doing its work.