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, t.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 (six) of extinct, 
than of living (five) 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 vertebre vary in being 
either biconcave, concave in front, concave behind, or flat 
at each end ; the cup-and-ball vertebre 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, 
cailed an amnion and allantois, envelop the embryo. 
