EVOLUTION AS SEEN IN THE STRUCTURE OF MODERN ANIMALS 21 



port the body in the air and, unHke the wings of an airplane, serve as the 

 means for forward propulsion. The reader can readily supply additional 

 examples from his own observation. When animals live similar lives they 

 usually resemble each other to some extent, the similarity being connected 

 with the similar functions which their bodies serve. Similarity of structure 

 connected with similarity of function is termed analogy; structures exhibit- 

 ing it are said to be analogous. 



Insects resemble birds and bats in the possession of wings. The insect 

 wing somewhat resembles a structure molded in plastic. Both the wing and 

 the outer covering (exoskeleton) of the body contain a complex material 

 (nitrogenous polyssacharide) called chitin. The wing is stiffened by a 

 series of hollow tubes, the "veins" (Fig. 8.21, p. 165). The whole forms a 

 lifeless structure operated by muscles attached to its base. 



The wings of bird and bat are quite otherwise (Fig. 3.1 ). The support- 

 ing surface of the bird wing is composed of feathers, that of a bat wing of 

 a membrane formed of modified skin. The feathers, in the one case, and 

 the membrane, in the other, are supported by an internal skeleton of bone, 

 a very different material from the chitin of the insect. The skeleton of 

 these wings forms a series of segments. The segment attached to the body 

 is supported by a single bone (Fig. 3.1), the humerus. To the free end of 

 the humerus two bones attach, the radius and ulna. Next comes a group of 

 little bones, the carpals (corresponding to man's wristbones), then the 

 metacarpals (corresponding to the bones in the palm of man's hand), and 

 finally the phalanges (corresponding to the bones in man's fingers). In the 

 bat the first "finger," corresponding to the human thumb, is short and 

 tipped with a claw, while the other four fingers have long slender meta- 

 carpals foiTning stiffening supports, like the ribs of an umbrella, for the 

 wing membrane. In the bird the carpals, metacarpals, and phalanges are 

 partly fused into an irregularly shaped bone serving to support the feathers 

 of that part of the wing. We see, then, that the wings of insects are really 

 very different from the wings of birds and bats. We may conclude that 

 analogous similarities are on the whole rather superficial in nature. 



Homology 



In describing the skeleton of the wings of birds and bats, in the preced- 

 ing paragraph, we have repeatedly referred to the skeleton of the human 

 arm to make our meaning clear. It will already be evident, therefore, 

 that considerable similarity exists between the skeletons of the arm of man 

 and of the wings of bird and bat. The similarity is particularly clear in the 



