hNTRODUCTION. 



5 



means of the pelvis. The fore-hmbs persist in all 

 members of the class, but the hind ones have dis- 

 appeared in the whales and sea-cows (Sirenia). 

 Hut this is only the result of a process of degen- 

 eration, and small bones, which are nothing else 

 than the rudiments of these undeveloped limbs, 

 can still be found hidden in the flesh of these 

 animals. 



The Shoulder-girdle. — What is known as the 

 shoulder-girdle is composed originally of three 

 bones, the shoulder-blade or scapula, the collar- 

 bone, and the coracoid. These bones are still 

 found separate in the monotremes as in the lower 

 fi\e-fingered xertebrates. But in all other mammals 

 the coracoid early becomes fused with the shoulder- 

 blade, of which it then forms a process. The 

 collar-bone is very variable in its development. 

 It might be called the bone of the specialized 

 function of the limb, the bone which enables the 

 limb to act in some particular way. It is, in fact, 

 present in all mammals in which the fore-limb 

 has to perform complicated functions, in which, for 

 exainple, it is cmplo_\'ed as a hand for grasping, as 

 a spade tor digging, or as a paddle in swimming; 

 but it becomes rudimentary or vanishes entirely 

 when the limb has merely to support the body in 

 walking and running. 



The Fore- and Hind-limbs. — The fore- and 

 hind-limbs correspond with each other as regards the 

 composition of their bony framework. The bone 

 of the upper arm (the humerus) corresponds to the 

 thigh-bone or femur, the radius to the fibula, the 

 ulna to the tibia, the wrist to the ankle, the meta- 

 carpus (the bones of the palm of the hand) to the 



In order to understand these transformations we 

 must go back to the primitive conditions which are 

 to be seen, on the one hand, in embr)os, and, on 

 the other hand, in the oldest ancestral forms known 

 to us. What do we then see.' 



The limbs of the embr)-o scarcel}- differ at all 

 from one another. In the embr)-o the fore-limb of 

 a bat, which is destined in the adult to support a 

 flying membrane, is exactly like the hind-limb, 

 wdiich ultimatel}' develops into a sort of paw with 

 fi\'e toes and cur\ed claws. The limb of a kangaroo 

 does not, in the first instance, difter in an\' way 

 from that of a monkey or a sheep. The initial 

 form is alwa}-s the same: a small lobe attached to 

 the side of the bod_\', and having developed on it 

 fi\'e ra}-s, the future toes, xxhich are connected 

 together do\Mi to their extremities b}- a membranous 

 continuation of the lobe. One of these rays, w hich 

 corresponds to the thumb or first digit (for we 

 alwa\-s count the digits beginning with the thumb 

 or great toe) stands a little apart from the other 

 four, each of Axhich has its axis corresponding to 

 that of the limb itself 



The original form of the mammalian limb is 

 accordingl)- in all cases that of a flipper, the thumb 

 of which stands a little apart, while the digits are 

 all connected together by the skin. 



From these facts we deduce the conclusion that 

 the webbed feet of the bea\ers, otters, seals, in short 

 of all mammals living in the water have preserved 

 the original type, \\hich has become specialized in 

 a particular direction, or, in simpler language, has 

 become adapted to a special use, in the fin of the 

 whales and sea-cows. 



If we trace the de\clopment of the flying-mem- 



metatarsus (those of the sole of the foot), the 



fingers finally to the toes; and in spite of the fact brane step b\- step in the embryo of the bat, we 



that the elbow and the knee have contrary directions j can easily be convinced that this member is only 



these joints are also homologous. I an aerial paddle, the structure of which has re- 



Now nothing is subject to greater variations than i mained essentially the same as that of the aquatic 



the structure of the limbs, which are primarih- : paddle of the seal. In the flying-membrane of the 



affected by adaptation to the most diverse modes 

 of life. Is it possible, indeed, to imagine organs 

 more difterent than the foot of a horse, the paw of 

 a dog, the hand of an ape, the fin of a dolphin, and 



bat the chief part of the supporting framework 

 consists of the greatly elongated bones of the fingers 

 or digits, but the motion of the membrane in the 

 act of fl\-ing is eftected bv means of the rest of the 



the wing of a bat.' And yet these limbs, so diverse ' fore-limb just as in the flipper of the seal, in which 

 in respect of their structure and function, are all i the same bones are present, onl\- not elongated 

 constructed on one and the same ground-plan; I and not spread out. Apart from the elongation of 



the\' are composed of the same elements, and the 

 final result has been brought about onl\- by 

 modifications and processes of reduction and sup- 

 pression originally not at all striking, but which 

 have gone on gradualh' accumulating. 



the digits and the extension of the membrane 

 necessitated by the gaseous element in which the 

 creature has to "swim," the wing of the bat is thus 

 exactly like the flipper of the seal. It is accor- 

 dingly onl)- a s\\imnnng-paddle special!}- de\'eloped 



