282 



NATURE 



[July II, 1878 



THE GENESIS OF LIMBS 



WHY are our limbs so much alike and yet so different ? 

 What do our limbs stand for as compared with the 

 bodies of other animals ? Whence have limbs such as 

 ours arisen ? What is a limb ? 



The word limb is the Anglo-Saxon word livi, most 

 probably connected with the Latin litnbus — the border, 



Fig. I. — A Centipede. 



outer edge, or extremity of anything, and thence applied 

 to any attached, projecting, or out-lying portion. 



But there are projecting portions of animal bodies 

 essentially like our own body which are not called limbs, 

 t.g., the dorsal and anal fins of fishes, while yet that name is 

 freely bestowed upon structures which have no relation to 

 our limbs save a relation of analogy from similarity of use, 

 as, e.^., the legs of insects or the arms of star-fishes. Insects 



Fig. 2. 



Fig. 3. 



Tig. 2. — Anterior (palmar) surface of the skeleton of man's hand — c, cunel- 

 forme : /, lunare ; tft, magnum ; m^, metacarpal of thumb ; m'^-in^, 

 metacarpals of the four fingers ; /, pisiforme ; F^, first phalanx of the 

 thumb and four fingers — i.e. of the five " digits ;" P^, second phalanx 

 of the five digits ; P\ third, or ungual phalanx ; sc, scaphoides ; t, 

 trapezium ; tz, trapezoides ; u, unclforme. 



Fig. 3. — Dorsum, or upper surface, of skeleton of right foot. — a, astragalus; 

 t , ento-cuneiforme ; c*, meso-cuneiforme ; c3, ecto-cuneif orme ; ca, cal- 

 eaneum; co, cuboides; h, distal phalanx of hallux; >«', metatarsal of 

 hallux ; vi^-mS, metatarsals of the four outer tees ; «, naviculare. 



and their allies present certain resemblances and differ- 

 ences carried to a higher degree than in us, and which 

 may be here adverted to. The difference in shape 

 between the limbs of the right and left sides in us is 

 minute and accidental. Our bilateral symmetry is com- 

 plete, but in many crustaceans the shapes of the right and 

 left great claws differ to a large extent. The resemblance 

 between the thoracic and pelvic limbs in us is great, but the 



resemblance between the successive legs of many arthropods 

 is much greater, especially in the class of centipedes, where 

 the successive segments of the body, with their appendages, 

 exhibit serial symmetry carried to the highest degree. The 

 amount of likeness, as regards serial sym7netry,'whxch. exists 

 between our pairs of limbs is less than exists in many 

 back-boned creatures, Avhile at the same time there are a 

 great many others in which it is not carried nearly so far 

 as it is in ourselves. These varying degrees 

 of serial symmetry are such that upon the 

 theory of evolution we must suppose that if 

 this serial symmetry originally existed, it 

 must have been lost and reacquired perhaps 

 several times to produce what we see before 

 us in the existing creation. 



Thus if we compare with the structure of 

 the human hand and foot the same parts in 

 apes, we find that in them the toes (or digits 

 of the foot), instead of being short like ours, 

 are long and mobile like our fingers, while 

 the great toe (or hallux) is set out at an angle 

 from the others, to which it is powerfully 

 opposable. At the same time the main 

 points of structure of the ape' s foot remain 

 like our own, and thus while it is morpho- 

 logically a foot, it is functionally more or less of a hand. 

 Here, therefore, serial symmetry is already more complete 

 than in us. 



If we descend to hoofed beasts, e.g, the hog, the giraffe, 

 or the horse, we find the number of digits equally and 

 simultaneously reduced in both the fore and the hind limb, 

 and while in the two former creatures the third and fourth 

 digits of each extremity are increased in size at the ex- 

 pense of the others, in the horse there is but one digit 

 so increased — the animal walking upon but four digits, 

 which answer respectively to our two middle fingers and 

 our two middle toes. 



Fig. 



Fig. 5. 



Fig. 4.— Right pectoral limb of a Giraffe.— tr;/, scaphoides; d3, proximal 

 phalanx of third digit; d*, proximal phalanx of fourth digit; g; mag- 

 num ; gt, great tuberosity of the humerus ; h, shaft of the humerus ; 

 l7i, lunare ; w/3t4, united metatarsals of third and fourth digits ; o, ole- 

 cranon ; pi, pisiforme ; r, radius ; sc, cuneiforme ; mi, unclforme. 



Fig. 5.— Right pectoral limb of Horse.— c, cuneiforme; /;, humerus; lu, 

 lunare ; m^, metacarpal of the third digit— the only one fully deve- 

 loped ; w4, rudimentary fourth metacarpal ; vig, magnum ; //, pisi. 

 forme ; /', proximal phalanx ; /^^ middle phalanx ; /3, third or ungual 

 phalanx ; s, sesamoid ; 7i)i, unclforme. 



It is, then, quite a mistake to regard the ox's hoof as 

 answering, morphologically, to the horse's hoof "cloven;" 

 each single hoof of the horse answers only to the inner 

 division of each double hoof of the ox or giraffe. Now in 

 all these creatures we find a still further increase in serial 

 symmetry as compared with the apes and man. 



