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 CHAPTER II. THE LIVING BIRD. Factors of Evolution. — If while in the fields we ob- serve birds with an appreciative eye, we shall soon be impressed with the great diversity shown in their struc- ture and habits. The Fish Hawk plunges from the air into the water and grasps its prey with merciless talons. The Hummingbird daintily probes a flower. The Wood- pecker climbs an upright trunk, props itself with its stiff, pointed tail-feathers, while with its chisel-shaped bill it excavates a grub and then impales it with its spearlike tongue. These birds tell us a wonderful story Fig. 2.— End of spearlike tongue of Pileated Woodpecker. (Mucli enlarged.) of adaptation to the conditions of life, and, knowing that they have descended from a common ancestor, we ask, " Why do they now differ so widely from one another ? " Biologists the world over are trying to satisfactorily answer this question, and it is impossible for me to even mention here all the theories which they have advanced. However, some knowledge of the most im- portant ones is essential if you would study the relation between the bird and its haunts and habits. The Dar- win-Wallace theory of Natural Selection, in more or less 14