THE VERTEBRATA . 231 



cartilaginous rays on both margins. If we compare this 

 with the leg of a newt, which shows the simplest form of 

 terrestrial limb, we see the essential changes in the latter 

 are (i) the lengthening of the axis ; (2) the introduction 

 in the second joint of a pair of elements, side by side, 

 which makes possible a movement of rotation impossible 

 in the fin ; and (3) the great reduction in number of the 

 rays and their separation to form five fingers or toes, 

 capable of independent movement (Fig. 66, a, b). 



The Reptilia of the present day consist of four or five 

 small orders the lizards and snakes, tortoises and 

 crocodiles being the familiar forms. To these must be 

 added, as representing a separate order, the New 

 Zealand lizard Hatteria, sole survivor of an important 

 Permian and Triassic series. The birds should also be 

 counted as another order. These are the few scattered 

 survivors of a series of forms so vast and varied that the 

 Mesozoic era in which they flourished has been called 

 the Age of Reptiles. They spread over the surface of 

 the continents, hitherto almost devoid of animal life, and 

 exhibited the same " radiative adaptation " to all possible 

 methods of life that the Mammals showed afterwards in 

 the Cainozoic era. There were herbivorous reptiles and 

 carnivorous reptiles, aquatic, terrestrial, and aerial reptiles. 

 We cannot here deal with the many instructive facts 

 shown by the study of fossil reptiles, but must confine 

 ourselves to the case of one order of reptiles, whose 

 remains are fairly common in some of the Jurassic strata 

 (Lias), the Ichthyosauria. These occupy the same posi- 

 tion among Reptiles that the whales do among Mammals. 



