242 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



that of flying. But they are not at all homologous. 

 They have not a common origin ; the one could not 

 have, and did not, come from the other. In a word, 

 they are not at all the same thing. But the wing of a 

 bird and the wing of a bat are homologous parts. Not 

 only so, but both are homologous with the pectoral fin 

 of a fish, the fore limb of a reptile or a mammal, and 

 the arm and hand of a man ; for all these, though now 

 so different in form and function, can be shown to have 

 a common origin by descent — to be, in fact, the same 

 thing, only modified for various purposes. 



2. The lungs of man and the gills of a fish are analo- 

 gous parts — i. e., they perform the same function in the 

 animal economy — viz., the aeration of the blood. But they 

 are not homologous. By no possibility could one have 

 come by modification out of the other. What, then, in 

 the fish is the organ homologous with the lung of man? 

 It is the air bladder. This is the organ which by modifi- 

 cation became the lung of air-breathing animals. The 

 proof of this is complete, for all the steps of the gradual 

 change can be traced. This is shown as follows: 



(a) In most typical fishes the air bladder is wholly 

 isolated and used only as a float. In such cases it is 

 colorless. But in some fishes it is connected by a slen- 

 der tube with the throat, and doubtless the contained 

 air is renewed from that source. In still other fishes it 

 is not only connected with the throat, but air is regu- 

 larly and voluntarily taken in. In such cases it is vas- 

 cular and therefore reddish in color. This is the case in 

 the gar pike (Lep'dosteus). This fish may be observed 

 to come at intervals to the surface and gulp down air, 

 which is again afterward expelled in bubbles. The gill 

 breathing is to some extent supplemented by air breath- 

 ing. Finally, in a few of the most reptilian fishes (Cerato- 

 dus and lepidosiren) the air bladder is not only vascular, 



