52 UNIVERSALITY OF DEGENERATIVE EVOLUTION 



FIG. 



The fifth toe next disappeared, leaving the foot 

 three-toed, as in the viscacha, the 

 capybara and agouti. In the foot 

 of the agouti the metatarsals have 

 considerably gained in length, and 

 are closely pressed together. 



In the jerboa (fig. 23) the three 

 metatarsals (M.) are united, and 

 the intervening tissues have dis- 

 appeared. Besides this modifica- 

 tion, the hind limbs of the animal, 

 being used for leaping only, have 

 become thinner, while gaining in 

 to. 23. Hind limb of length by the elongation of the 



Dipus (Jerboa). I, first J 



rudimentary digit ; II, metatarsals. 1 

 second digit; III, third 



digit; iv, fourth digit; (&) The Kangaroo. Although 



M t bone formed by v ' 



fusion of 2nd, 3rd, and now inhabitants of the plain, 



4th metatarsals; ca, r 



caicaneum; as, astra- kangaroos are descended from 



gulus; re, navicular; 



cs, third cuneiform ; C 2, arboreal marsupials, which them- 



second cuneiform; cb, 



cuboid. (After Flower, selves have gradually ceased to be 



Introduction to the Oste- D '' 



oiogy of the Mammalia.^ arboreal. 2 All the intervening 

 phases of this transition from type to type are known. 



1 The last segment of the leg of the jerboa the foot being 

 formed by the union of the second, third and fourth metatarsals 

 corresponds more or less to the formation of the bird's foot. 

 This, however, is a case of convergent modification more apparent 

 than real, for in the cannon bone of birds, besides the second, third 

 and fourth metatarsals, there is the whole set of distal tarsals, 

 whereas in the jerboa the distal tarsals remain free and isolated 

 bones. 



2 L. Dollo, autographs of the course given at the Institute of 

 Solvay, 1891-1892, 5th lecture, The Origin of Kangaroos. 



