CHAPTER IV 

 STRUCTURE AND CLASSIFICATION OF REPTILES 



The two great groups of Backboned Animals so far considered, 

 i.e. Mammals and Birds, are both characterized as "warm-blooded", 

 for the animals included possess hot blood which is maintained 

 at a practically constant temperature independently of external 

 conditions. This peculiarity is associated of necessity with very 

 perfect breathing arrangements (see pp. 45 and 147), and results 

 in great activity and high intelligence. 



Contrasted with these two high groups we have the Reptiles 

 and still lower forms, which are " cold-blooded ", in the sense that 

 the blood is of the same approximate temperature as the sur- 

 rounding medium — ^air or water, as the case may be — and is 

 therefore hot in hot weather and cold in cold weather. Such 

 animals, as compared with Mammals or Birds, are in the main 

 sluggish, and not remarkable for intelligence. 



Reptiles are a very ancient group, and probably form the stock 

 from which both Mammals and Birds have been derived, on which 

 subject more will be said in another section of the book. Reptiles 

 preceded Mammals as the dominant Vertebrates both on sea and 

 land, and many remarkable groups once existed which are now 

 entirely extinct. Recent Reptiles include crocodiles, lizards, turtles, 

 and snakes, with their respective allies. The most average forms 

 are probably Lizards, of which there are three British species, 

 and a brief description of the most typical of these will be an 

 appropriate introduction to the study of Reptiles in general. 



The Sand Lizard {Lacerta agilis) is commonly to be found on 

 sunny slopes in this country, and appears to be especially abundant 

 where the soil is of a sandy nature. When thoroughly warmed 

 by basking, it is capable of moving with considerable rapidity, 

 hence the specific name agilis. Average specimens are not more 

 than 5 or 6 inches long, but this length is sometimes exceeded by 

 2 or 3 inches. 



