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 FIG. 2. Bones 



. p , , , 



bones ot 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 



of left 



flipper of Greenland 



Right whale. Letters 

 l ' (From 



