24 LECTUEES ON BIOLOGY 



and instead of being jointed are held rigidly together by strong 

 bands. The hind extremities, however, being relieved of their 

 functions through the immense lengthening of the body and the 

 formation of a fleshy tail-fin, have in most species degenerated 

 without leaving a trace behind. Only in the Greenland Whale 

 (Balcena mysticetus) and a few others do we find hidden away 

 in the flesh and unconnected with the vertebral column the 

 insignificant remains of a short thigh and leg. 



As Uria troile forms the transition from the air to the 

 water, so are the seals (Pinnipedia) the transition form between 

 land and sea. We still find in them four . strongly developed 

 extremities which have been transformed into fins. The 

 pectoral fins not only contain internally the complete parts 

 of the mammalian arm, but have also five fingers which are 

 armed with claws and firmly jointed together by exceedingly 

 strong webs. The posterior fins, which also clearly betray their 

 origin, are turned backwards and fulfil in swimming the function 

 of a caudal fin: they serve as a rudder. Locomotion on land 

 is effected chiefly by an alternate lowering and raising of the fore 

 and hind part of the body and an eel-like bending and stretching 

 of the whole body. The legs are only used for support in 

 climbing inclines. 



Among the many other known instances it will only be 

 possible to mention the Mole (Talpa europoea), whose short and 

 strong arms, combined with a hand armed with sharp shovel- 

 claws, seem admirably adapted for digging his long subterranean 

 runs. It is curious to observe that similar habits of life have 

 brought about a similar development in the forelegs of an insect, 

 the Mole-cricket (Gryllotalpa vulgaris). 



The conquest by mammals of the elements became complete 

 with their conquest of the air. The hair of mammals, as com- 

 pared with feathers, does not seem adapted to flight, but Nature, 

 never at a loss, knows how to overcome this difficulty, and thus 

 we see bats and flying-foxes share with birds the ' liberty of the 

 air.' In the absence of feathers the skin is used for the forma- 

 tion of a flying-apparatus. This necessitates a radical transfor- 

 mation of the arm-skeleton ; it is not enough that the single 

 bones of the anterior extremities are enormously elongated, but it 

 is also necessary to provide a strong frame on which the delicate 

 flying-membrane may be expanded. This task is solved by a 



