EVIDENCE FROM COMPARATIVE ANATOMY 47 



no true joint between any of the bones, which ar- 

 rangement allows but a very limited motion, merely 

 enough for the necessary flexibility of the paddle. 

 The arm-bones are short, mas- 

 sive and simple and, what is 

 extraordinarily rare among mam- 

 mals, the number of joints in 

 the digits is much increased, 

 to give the needed length 

 and support to the flipper. 

 Clearly, the whale's paddle is of 

 precisely the same type as the 

 fore limb of a land quadruped, 

 but transformed and adapted to 

 a purely marine habit of life. 



The comparison of a bat's 

 wing with that of a bird brings 

 to light two quite different modes 

 of adaptation of similar elements 

 to the purposes of flight, and in 

 both groups we find certain dif- 

 ferences in the completeness of 

 adaptation. In the bat the 

 bones of the arm are very slender 

 and greatly elongated; the ulna 

 is much reduced, only the upper 

 third remaining and this is co-ossified with the radius. 

 The first digit, or thumb, is divaricated from the 

 others, is not included in the wing-membrane and 

 bears a curved, hook-like claw. The bones of the 



Fig. 2. Bones of left 

 flipper of Greenland 

 Right Whale. Letters 

 as in Fig. 1. (From 

 Allen.) 



