July II, 1878] 



NATURE 



28 



If, however, we turn to such an animal as the mole we 

 find a much decreased degree of such symmetry, the fore- 

 Ihnb being of enormous strength, with its bones shortened 

 and broadened out, while the hind-limb is slender and deli- 



FiG. 6. 



Fig. 7. 



Fig. 6.— Right Fpelvic pimb [o£ Giraffe. — a, astragalus ; c, calcaneum ; cu, 

 cuboides; </3, proximal phalanx of third digit ; </■♦, proximal phalanx of 

 fourth digit ; /; femur ; /' , rudiments of fibula (the line is not continued 

 far enough — the rudimentary fibula is a small ossicle reposing on the 

 upper surface of the calcaneum, as shown in the figure) ; >«3+4, meta- 

 tarsals of digits 3 and 4 united into one " cannon-bone " ; /a, patella; 

 t, tibia. 



Fig. 7. — Skeleton of right {pelvic limb of Horse. — a, astragalus; c, cal- 

 caneum ; cu, cuboides ; ec, ecto-cuneiforme ; f, femur ; gt, great tro- 

 chanter ; mi, metatarsal of third digit ; w«4, rudimentary fourth meta- 

 tarsal ; «, naviculare; pa, patella; j>', /», and /^^ first, second, and 

 third phalanges of the third and only digit ; s, sesamoid ; t, tibia ; ti, 

 third trochanter. 



cate. The mole works underground with such exceeding 

 rapidity that it has been said to fly beneath the soil, but 

 in the beast which really does fly — the bat— serial sym- 



-nt^ 



Fig. 8.— Hand of Bat {Pteroj>ui).—m^-tn^, metacarpals of the" four fingers ; 

 p, pollex, with a very short metacarpal ; sc, scaphoides. 



metry is still less developed. The framework of the bat' s 

 wing consists of the very same bones which exist in the 

 human arm and hand, only exceedingly elongated and 

 slender. The four fingers — wonderfully drawn out — are 



connected together (and to the body and legs) by a deli- 

 cate web of skin. The foot is a striking contrast to the 

 enormously enlarged hand, being small in size with short 

 toes. And yet, though serial symmetry is thus disguised in 

 the bat, it nevertheless shows itself in other ways more or 

 less noteworthy. The outer bone of the fore-arm — the ulna, 

 is incompletely developed, and the corresponding bone of 

 the leg — the fibula, is also incompletely developed. But 

 much more than this, in some bats we find outside the 



Fig. 9. — Left foot of a Monitor Lizard {Varamts).—/, fibula ; m^-mi, the 

 five metatarsals, tn^ being that of the hallux; /, tibia; i, astragalo-cal- 

 caneum ; 2, cuboides ; 3, ecto-cuneiforme. 



elbow-joint a distinct and separate little bone which quite 

 answers to the knee-pan (or patella) of the leg — a most 

 exceptional case of serial homology. 



The creatures just referred to are all mammals, but 

 birds and reptiles present us with some instructive exam- 

 ples both of serial homology and discrepancy. In our- 

 selves and in all beasts, the motion of the foot upon the 

 leg takes place between the long bones of the latter 

 (tibia and fibula) and the tarsus. In the crocodile, or 



Fig. 11. — Right foot of Emeu.— a, astragalus; d^-di, second, third, and 

 fourth digits ; in, metatarsals anchylosed together except at their distal 

 ends ; /, tibia ; /21 distal tarsal element. 



monitor, it is not so, but the upper part of the ankle, or 

 tarsus (answering to our astragalus and os calcis), is 

 firmly and immovably fixed to the leg bones, while the 

 lower part of the tarsus is firmly fixed to the metatarsals. 

 Thus in the crocodile, or monitor, motion does not take 

 place between the whole ankle and the leg, but in the 

 middle of the ankle (or tarsus) itself. 

 In the leg of a bird there at first sight seems to be no 



