94 PRIMITIVE ANIMALS 



The curious thing about the true land Vertebrate 

 limb is that it appears apparently for the first time 

 in the Amphibia in its complete and fully formed 

 condition, consisting in each limb of a single bone, 

 the burner us or femur, jointed to the pectoral or 

 pelvic girdle respectively, followed by two parallel 

 bones, the radius and ulna in the fore limb, the tibia 

 and fibula in the hind limb, and then come a number 

 of small bones forming the wrist and ankle, and 

 finally five digits, the fingers and toes. That is the 

 uniform structure of the pentadactyle limb of the 

 terrestrial Vertebrate, which appears suddenly, to all 

 appearances from nowhere, and is preserved bone for 

 bone with extraordinary uniformity in all the land 

 Vertebrates, including man. 



Of course we must believe that this pentadactyle 

 limb had an origin, and that it was somehow derived 

 from the fishes' fin, but there is no shadow of a 

 suggestion, either in living forms or in the geological 

 record, of the steps by which this fundamental ac- 

 quisition was arrived at. 



Let us now turn to the consideration of the origin 

 of the breathing organs of the land Vertebrates. 

 The typical fish obtains its oxygen from the gas 

 dissolved in the water, a fresh stream of water 

 being continuously passed over the gills, the walls of 

 which are very thin and vascular, so that the 

 blood circulates in them in close proximity to the 



