FROM THALES TO LAMARCK 23 



lengthening of the wing, in the penguin the single bone-parts 

 have become shorter and stronger, and flattened, in order to 

 provide a broad 'rowing-surface.' (See fig. 5, 3 and 4.) 



There are other birds which provide, as it were, the transition 

 from the aerial to the aquatic life. The wings of the Common 

 Guillemot (Uria troile), a bird of the Northern Seas, often found 

 in Heligoland hatching its eggs, show a distinct shortening, the 

 bones a broadening. But these birds still possess well-developed 

 wings and use them for short-distance flights. They derive, 

 however, greater advantage from their wings on their extensive 

 swimming tours on and under the water by using them as oars. 



Another step further has been taken by the Great Auk (Alca 

 impennis), once very common in the Northern Sea, but now 

 almost extinct, owing to the senseless persecution of this bird 

 by sailors. Its fish-like wings still bear feathers, but they are 

 much too short for flight and serve now only for locomotion in 

 the water. 



The best proof for the statement that it is the function w r hich 

 determines the form of all organs is supplied by animals of 

 different classes but with the same life-habits. Whales and 

 dolphins are even to-day regarded by many as fishes, so com- 

 pletely has their shape been altered by a continual sojourn in the 

 water. Their spindle-like body, the absence of hair, the strong 

 tail terminating in a double fin, the formation of an unpaired 

 dorsal and paired pectorals, and finally the absence of a neck 

 make this mistake pardonable. But the zoologist knows that 

 whales and dolphins are mammals. They are viviparous and 

 suckle their young. They have warm blood and breathe air 

 through lungs like other mammals. There are, however, sharp 

 distinctions. All other mammals have two pairs of extremities, 

 but dolphins and whales have only two pectoral fins, which may 

 perhaps be compared to forelegs ; of a second pair, however, 

 there is no external trace. But as whales have been gradually 

 transformed from mammals into denizens of the deep we find on 

 examination that the breast fins, while externally resembling the 

 unjointed fins of fishes, are internally supported by the same 

 skeleton-part as are, for instance, the arms of man or the fore- 

 leg of a lion. (Compare fig. 5.) But in order to obtain a strong 

 ' rowing-surface ' the bones are much shortened and flattened, 



