50 REVIEWS—A MANUAL OF GEOLOGY. 
the volume, Historical Geology, contains a detailed exposition of the 
geological ages and epochs, from the far distant Azvic period to the 
present or historic time; and it exhibits in this connexion, many im- 
portant generalizations respecting the life-forms, and the various 
climatic, geographical, and other features, of the eras thus discussed. 
To this succeeds a section on Dynamical Geology, or review of the 
yarious forces now in action in modifying the surface and general 
conditions of the globe. 
Such is a brief synopsis of the contents of this important volume. 
On account of the peculiar nature of the work, its constant refer- 
ence to back pages and sections, and the necessarily paragraphic 
form in which much of its information is laid before the reader, it is 
not easy to obtain an extract for quotation, more especially from the 
purely geological portions, in which its able character can be fairly 
shewn. The subjoined passages (selected partly on account of their 
general interest) may serve, however, to shew the uninitiated reader 
that a treatise on geology comprises something more than a mere 
dry description of barren rocks and stones: 
Criteria of Rank among Animals. 
(1.) Under any type, water-spectes are inferior to land-species: as the Seals to 
the terrestrial Carnivores ; the water-articulates or Worms and Crustaceans to 
land-articulates or Spiders and Insects. 
(2.) Species of a tribe bearing some of the characteristics of an infertor tribe or 
class are inferior species, and conversely.—Thus, Amphibians show their infe- 
riority to True Reptiles in the young having gills like Fishes ; the early Theco- 
dont Reptiles, inferiority to the later in having biconcave vertebre, like Fishes ; 
the Marsupials and Hdentates, inferiority to other Mammals in having the 
sacrum consisting of only two united vertebra, as in most Reptiles. On the 
cortrary, the Dinosaurs show their superiority to other Saurians in having the 
sacrum made of five (or six) vertebre, as in the higher Mammals. 
(3.) As a species in development passes through successive stages of progress, 
relative grade in inferior species may often be determined by comparing their struc- 
tures with these embryonic stuges.—As a many jointed larve without any distinc- 
tion of thorax and abdomen is the young state of an Insect, therefore Myriapods 
or Centipedes, which have the same general form, are inferior to Insects. Asa 
young living Gar has a vertebrated caudal Jobe (making an accessory upper 
lobe to the tail), which it loses on becoming adult, therefore the older Ganoids 
with vertebrated tails (or heterocercal) are inferior to the latter in which the 
tails are not vertebrated (or are homocercal). As the young of a Frog (a tad- 
pole) has the tail and form of a Salamandrian, therefore the Salamandrians are 
inferior to Frogs. As the number of segments in the young of Insects often — 
exceeds much that of the adult, therefore species of adult animals in which there 
