LIMBS 



125 



in accordance with this view, which is further supported by many 

 of the facts stated above and by numerous others relating to the 

 structure and development of the fins in Fishes, as well as by a 

 study of such fossil forms as the Palaeozoic Cladoselache. 



GENERAL CONSIDERATIONS ON THE LIMBS OF THE 

 HIGHER VERTEBRATA. 



It thus appears possible to derive the skeleton of the fin of all 

 the orders of Fishes from a single ground-type, but to trace the con- 

 nection of the latter with the extremities of Amphibia and Amniota 

 is a far more difficult task. Between these two types of limb there 

 seems to be a wide gap, in consequence of the different conditions 

 of life existing between aquatic and terrestrial Vertebrates : we do 

 not know how the limb of an air- 

 breathing Vertebrate (cheiroplery- 

 yiuvi), adapted for progression upon 

 Jand, has been derived from the fin 

 (ichthyopterygium), only fitted for use 

 in the water. 



Palaeontology furnishes no answer 

 to this question ; we know of no 

 fossil intermediate forms of limb, 

 and the various hypotheses which 

 have been put forward on the sub- 

 ject cannot be discussed here. We 

 may suppose that when the primitive 

 Amphibian first began to take on a 

 terrestrial mode of life, its fin, which 

 is practically a single-jointed lever, 

 amply sufficient for the movement of 

 the body in a fluid medium, became 

 gradually transformed into a many- 

 jointed system of levers. 



In other words, as the function of the limb was no longer 

 simply to propel the body, but also to lift it up from the 

 ground, the firmly-connected elements of the skeleton of the 

 tin gradually became loosened from, and placed at an angle 

 to, one another (knee, elbow), definite articulations being 

 formed between them in a .proximo-distal direction. Moreover, 

 the extremity must have changed its position with regard to the 

 body, so that, instead of projecting horizontally outwards, it became 

 bent downwards, and thus the angle between it and the median 

 plane of the trunk was gradually reduced, until in Mammals even- 

 tually, the longitudinal axis of the limb, when at rest, oame to lie 

 parallel with the median plane of the body. In the higher types 

 this is more particularly the case as regards .the posterior ex- 



FIG. 104. DIAGRAMMATIC FIGURES 



TO SHOW THE RELATIONS OF THE 



ANTERIOR FREE EXTREMITY TO 

 THE TRUNK IN FISHES (A), AND 

 THE HIGHER VERTEBRATES (B). 



S, pectoral arch ; Mt, metaptery- 

 gium ; Rd, radialia in A, radius 

 in B ; Ul, ulna ; proximally to 

 Ul and Rd is the humerus. 



