REPTILES. 489 



circumscribed as the modern birds, is without links forming 

 genetic bonds allying them all together. In fact, the different 

 classes of Vertebrates, as well as of other branches of the 

 animal kingdom, form an ascending series, from the more 

 generalized, though not always simple forms (numerous 

 groups comprising synthetic types), to those which are more 

 specialized, i.e., in which separate organs or groups of or- 

 gans are elaborated and worked out in great detail. This is 

 the tendency all through nature, and were Cuvier himself 

 now living, and were he to examine the facts revealed since 

 his death, he would, as many others in advanced life have 

 done, cast aside the limited, analytical notions of the past, 

 based as they were on fragmentary evidence, and adopt 

 the more philosophical principles of classification, based on 

 sciences that were in embryo thirty or forty years since. 

 These reflections have great force in the study of a class like 

 the reptiles where there are a larger number (five) of extinct, 

 than of living (four) orders, and where the fossil types were 

 of a more general, almost embryonic type, and consequently 

 gigantic and ill-shapen, showing a tendency to extremes or 

 prematurity in development rather than to an equality in and 

 maturity of the whole organization compared with their de- 

 scendants. A high degree of specialization of type tends 

 nearly always in living beings, plants as well as animals, to a 

 condensation and higher grade of form. These animals also 

 have given a name to the Age of Reptiles, the middle or 

 Mesozoic age of the world, when they were the dominant type 

 of life. 



The essential characters of reptiles are the following : As 

 regards the skeleton, the bodies of the vertebrae vary in being 

 either biconcave, concave in front, concave behind, or flat 

 at each end ; the cup-and-ball vertebrae are most common, 

 forming a strong and flexible joint well fitted for general 

 motion. The ribs are well developed, the sternum is rhom- 

 boidal ; there are usually, if not always, more than three 

 toes. The body is covered with scales ; the blood is cold, the 

 heart has in the crocodiles, the highest order, four chambers ; 

 two or more aortic branches persist, and certain membranes, 

 called an amnion and allantois, envelop the embryo. 



