group of animals which exceeds the birds in varied and suggestive material for the evolutionist. It is a significant fact that the birds, which appeared to Cuvier and his contemporaries a closed type, a group that seemed to fulfill the ideal conception of a class archetype, as compared with other groups which had their open as well as obscure relationships, should be of all groups the one that first yielded its exclusive characteristics. In fact, there is no group in which the barriers have been so completely demolished as in this apparently distinct and isolated class. An attentive and patient study of the birds has established almost every point defined by Darwin in his theory of natural selection. One has only to recall the marked reptilian affinities as shown in their embryological and paleontological history. Besides all these structural relationships, the birds possess as a group remarkable and striking illustrations of variation in color, size, marking, nesting, albinism, molting, migration, song, geographical variation, sexual selection, secondary sexual characters, protective coloring; and in their habits show surprising mechanical cunning and ingenuity, curious and inexplicable freaks, parental affection, hybridity indeed, the student need go no further than the birds to establish every principle of the derivative theory.
The many observations on the nesting habits of birds would form a curious chapter as illustrating the individual peculiarities of these creatures.
Dr. A. S. Packard[1] records the fact, as related to him by Mr. Wyatt, of wild geese nesting in large cottonwood-trees on Snake River, west of the Rocky Mountains; and Dr. Coues, in his "Birds of the Northwest," says wild geese "nest in various parts of the Upper Missouri and Yellowstone regions in trees." Mr. H. W. Turner[2] observes a robin nesting on the ground. The late Dr. T. M. Brewer[3] points out some very curious "variations in the nests of the same species of birds." He not only observes individual variation in nest-structure, but shows that in different regions of the country birds of the same species build different kinds of nests, and in reflecting on these peculiarities he is led to say, "If we can not understand what it can be that stimulates an Empidonax in Staten Island to build a pensile nest, while its fellow in Indiana builds one like a deep cup and surrounded with thorns, and another group in Pennsylvania put theirs on an exposed tree-top, and so flat that the eggs seem liable to roll out, we must see that some cause, hidden to us, is gradually effecting changes that sooner or later may become universal in the species, though which it is to be we may not be able to imagine."
Mr. J. A. Allen,[4] in writing on the inadequate theory of birds' nests, shows grave and important exceptions to Wallace's theory, though he subscribes heartily to his philosophy of birds' nests. He
