LIMBS. 101 



description holds good for the pectoral fin only of Elasmo- 

 branchs; the pelvic fin, however, is formed on a similar plan, but 

 remains in a lower stage of development, which is mainly expressed 

 by a limitation in the number of its basal segments. Thus a meso- 

 pterygium is not developed, and the propterygium is more or less 

 rudimentary, the inetapterygium being in this case also the most 

 important element. In male Elasmobranchs, a cartilaginous ap- 

 paratus the skeleton of the " claspers " is connected with the 

 metapterygium (comp. p. 327). 



In Ganoids, and still more in Teleosteans, the essential 

 plan of the cartilaginous portions of the fins may be derived from 

 that of Elasmobranchs ; the primary skeleton of the fins, however, 

 undergoes a considerable reduction, and, in consequence of the 

 appearance of membrane-bones in connection with it, a primary 

 and a secondary skeleton may be distinguished. 



The skeleton of the fins of Siluroids, Cyprinoids, and Gymno- 

 tida3, amongst the Teleostei, comes nearest to that of Ganoidei, 

 that of the Gymnotida3 being the most primitive of the three. 



GENERAL CONSIDERATIONS ON THE LIMBS OF THE 

 HIGHER VERTEBRATA. 



Though it is easy to derive the skeleton of the fin of all the 

 orders of Fishes from a single ground-type, it is far more difficult 

 to trace the connection of the latter with the extremities of 

 Amphibia. Between these two types of limb there appears to 

 be a wide gap, in consequence of the different conditions of life 

 existing between Fishes and Amphibians ; and the question thus 

 arises In what manner has the limb of an air-breathing Verte- 

 brate, adapted for progression upon land, become derived from- the 

 fin, only fitted for use in the water? 



Palaeontology furnishes no answer to this question ; we know of 

 no fossil intermediate forms of limb, and it is at present, therefore, 

 only possible to suggest a hypothesis on the subject. We may 

 suppose that when the primitive Amphibian first began to take on 

 a terrestrial mode of life, its fin, which we may look upon as a 

 single-jointed lever, and which amply sufficed 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 forwards, but also at the same time 

 to lift it up from the ground, the firmly connected elements 

 of the skeleton of the fin gradually became loosened from, and 

 placed at an angle to, one another (knee, elbow), definite articu- 

 lations 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 



