Young mammals and birds, before their clothing has well formed, are naturally susceptible to cold; this leads to the first genuine approach to a home among animals lower than man. Birds lay their eggs long before the creatures inside of them are ready to emerge. Accordingly they have learned to build nests in which to place these eggs, and to protect them from the outside air; meanwhile the bird keeps the eggs warm by close contact with its own body. The lowest of the birds may lay their eggs simply on the ground without any special protection. As we rise in the scale of the bird world we find nests provided for the eggs. These nests become increasingly complex and specialized, until we reach the oriole's home with its wonderfully woven mass of fiber, which, in spite of its apparent looseness, supports well the weight of the mother bird and of her eggs. The robin, not content with making a woven basket, plasters it with clay, and makes an absolutely impervious nest. When we remember that both mammals and birds are the modern descendants of cold and scaly reptiles of an earlier geological time, it becomes interesting to compare their clothing. Evidently in the mammals hairs began to come out between the scales. Gradually the scales became fewer and the hairs more abundant until finally the scales have all disappeared, except those that remain as the claws on the toes. The ancestors of the birds, on the other hand, boldly transformed their scales into feathers. Another need for shelter arises in connection with the approach of winter. This problem of withstanding the cold season is complicated by the presence of two new factors. First and most directly, the cold itself is a distinct obstacle to the comfort of many of these creatures; as a secondary result of this cold, the food of many animals disappears entirely in winter. Most of our birds meet this difficulty by changing their base of operations. When the north grows cold these creatures fly to the south. Some of their migrations cover enormous stretches of country. Our bobolink, so well known and loved by all watchers of spring migrations, passes twice a year between the latitude of New York and Rio Janeiro. One of our most careful students of bird migration says that the Golden Plover makes, twice each year, the long journey from the Arctic shores of North America to the plains of La Plata.