Dec. 25, 1873J 



NATURE 



M9 



from a common progenitor of frogs and of Chelonians 

 through a line of ancestors which never lost this cranial 

 shield, though the ancestors of all other beasts, all birds, 

 and all reptiles, except turtles, did lose it. It is incon- 

 ceivable, for if it were true a variety of the lowest 

 mammals (Marsupials* and Monotremesf) must have 

 less diverged from the ancient common siock than have 

 the members of the Rodent ordei', and nevertheless these 

 lowest mammals exhibit no trace whatever of such a 

 cranial shield. 



Here then we have an undoubted example of the inde- 

 pendent origin of structures so similar that at first sight 

 their similarity might well have been deemed a conclusive 

 evidence of affinity. 



Here, also, we have a memorable caution against hasty_ 



The bones of the fingers show, moreover, a greater 

 likeness, in certain respects, to those of beasts than to 

 those of reptiles. No finger has a greater number of 

 joints than three, while, in some lizards, the fourth digit 

 may have as many as five joints. 



In the frog the wrist-bones (called respectively the 

 magnum and unciforme) which support the third, fourth, 

 and the little fingers, are formed together into a single 

 ossicle. The same condition, however, sometimes occurs 

 even in the orang. On the other hand, the single bone 

 which in man and beasts supports both the "ring" and 

 the " little " fingers, may be represented by two ossicles in 

 the frog's class (or e.g. in Salamandm) and in some rep- 

 tiles (as in eg. Chelydrd). 



inferences from structural similarities. If this resem- 

 blance and that of the dorsal shields are, when taken 

 together, no signs whatever of special genetic affinity — it 

 is difficult to say what structural likenesses are to be 

 deemed unquestionable evidences of a common ancestr)'. 

 Passing now to the skeleton of the limbs, we come to a 

 character of great significance, as it is one which serves 

 to distinguish all the limbed species of the frog's class 

 from lower vertebrates. The character is very significant, 

 because all Batrachians, in spite of their numerous and 

 important fish affinities, differ from all fishes, and agree 

 with all higher classes in that they — if they have limbs at 

 all— have them divided into those very typical segments 

 wliich exist in man ; namely, shoulder-bones, arm-bones, 

 wrist-bones, .md hand-bones ; and ir.to haunch-bones, leg- 

 bones, ankle-bones, and foot-bones respectively. It is 

 difficult, then, to avoid the belief that in the Batrachian 



Fig. 46.— Lateral > 



showing bony lamella; behind 



class we come upon the first appearance of vertebrate 

 limbs, differentiated in a fashion which thenceforward 

 becomes universal. 



The bones of the wrist in the frog, again, present a 

 nearer resemblance to those in man than do those of 

 most reptiles, and this is still more the case in some other 

 members of the frog's class, e.g. Salainaudra and other Efts. 

 Nevertheless, there are certain reptiles, and, strange to say, 

 they are once more Chelonians, which agree in this resem- 

 blance—as may be seen in the hand of the tortoise— 

 Chelydnx serpentina. 



* i.i'. opossums, kangaroos. &<'■ 



1 The Ornithorhynchus and Echid a. 



diseal 

 and 5, 

 ;. 43.- 



iurfaCG ofsktleton ofriglu luuiil of the Tortoise, Chelydia 

 Gegenbaur). c, cuneiforme: in, inlcrmcdiumCorcentrale) : /, kmare: 

 5, metacarpals ; ;•, radius ; s, sciphoides ; «, ulna ; 1—5, the five 

 carpals, namely — i, trapezium ; z, trapezoides : 3, magnum ; 4 

 divided unciforme. 

 ■Outer side of right OS innominatum of Man. 

 or inferior f-pinous process of the ilium ; as, ai 

 IS process of the ilium : c, crest of the ilium ; ij 

 obdurator foramen ; /, pubis— its horizont 

 inferior spinous process : ps, posterior superior spir 

 of the ischium : t, tuberosity of the ischium. 



acetabulun 



i proce: 



No member of the frog's class which has an arm at all, 

 bears less than two fingers (as does Proteus) upon it. 

 Thus we meet with a number as small as that which is 

 developed amongst beasts in rumiuants, but not so small 

 a number as in the horse. 



In the rudimentary condition of its thumb the frog 

 participates in a very common defect, since this member 



Fig. 49 —Right side of Pelvis of Frog. i7, ilium ; is, ischium : f, pubis 



The three bones meet nt the uppper margin of the acetabulum. 

 Fig. 50.— Dorsal view of pelvis of Frog, showing the narrow ends of the 



ilia for attachment to the backbone, and also the close approximation of 



the acclabula. 



is absent in very many forins. It is so even in creatures 

 as highly organised and as like nnan in bodily structure 

 as monkeys, since both the spider- monkeys of America 

 and certain long-tailed monkeys {Colobi) of Africa, are 

 thumbless. 



In man, when standing, the weight of the body 

 is transferred to the limbs by a large bony girdle, 

 \yhich, from its basin-like shape, is called the ptlvis, 



