806 THE POPULAR SCIENCE MONTHLY. 



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 arche- 

 type, 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 rep- 

 tilian 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, geographi- 

 cal variation, sexual selection, secondary sexual characters, protective 

 coloring ; and in their habits show surprising mechanical cunning and 

 ingenuity, curious and inexplicable freaks, parental affection, hybrid- 

 ity 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* 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 f observes a robin nesting on the ground. The late Dr. T. M. 

 Brewer J points out some very curious " variations in the nests of 

 the same species of birds." He not only observes individual varia- 

 tion in nest-structure, but shows that in different regions of the coun- 

 try 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 under- 

 stand 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 Pennsyl- 

 vania 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 gradu- 

 ally 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, * 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 



* " American Naturalist," vol. xii, p. 54. + Ibid., vol. xii, p. 53. % Ibid., vol. xii, p. 35. 

 * " Bulletin of the Nuttall Ornithological Club," vol. iii, p. 25. 



