INTRODUCTION. 



means of the pelvis. The fore-h'mbs persist in all 

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

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

 But 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 

 five-fingered vertebrates. 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 

 example, it is employed as a hand for grasping, as 

 a spade for 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 tibia, the 

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

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

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

 fingers finally to the toes; and in spite of the fact 

 that the elbow and the knee have contrary directions 

 these joints are also homologous. 



Now nothing is subject to greater variations than 

 the structure of the limbs, which are primarily 

 affected by adaptation to the most diverse modes 

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

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

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

 the wing of a bat.'' And yet these limbs, so diverse 

 in respect of their structure and function, are all 

 constructed on one and the same ground-plan; 

 they are composed of the same elements, and the 

 final result has been brought about only by 

 modifications and processes of reduction and sup- 

 pression originally not at all striking, but which 

 have gone on gradually accumulating. 



In order to understand these transformations we 

 must go back to the primitive conditions which are 

 to be seen, on the one hand, in embryos, and, on 

 the other hand, in the oldest ancestral forms known 

 to us. What do we then see.' 



The limbs of the embryo scarcely differ at all 

 from one another. In the embryo the fore-limb of 

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

 flying membrane, is exactly like the hind-limb, 

 which ultimately develops into a sort of paw with 

 five toes and curved claws. The limb of a kangaroo 

 does not, in the first instance, differ in any way 

 from that of a monkey or a sheep. The initial 

 form is always the same: a small lobe attached to 

 the side of the body, and having developed on it 

 five rays, the future toes, which are connected 

 together down to their extremities by a membranous 

 continuation of the lobe. One of these rays, which 

 corresponds to the thumb or first digit (for we 

 always count the digits beginning with the thumb 

 or great toe) stands a little apart from the other 

 four, each of which has its axis corresponding to 

 that of the limb itself. 



The original form of the mammalian limb is 

 accordingly 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 beavers, otters, seals, in short 

 of all mammals living in the water have preserved 

 the original type, which 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 development of the flying-mem- 

 brane step by step in the embryo of the bat, we 

 can easily be convinced that this member is only 

 an aerial paddle, the structure of which has re- 

 mained essentially the same as that of the aquatic 

 paddle of the seal. In the flying-membrane of the 

 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 flying is effected by means of the rest of the 

 fore-limb just as in the flipper of the seal, in which 

 the same bones are present, only not elongated 

 and not spread out. Apart from the elongation of 

 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 only a swimming-paddle specially developed 



