REVIEWS—A MANUAL OF GEOLOGY. 51 
is an excessive number of segments (beyond the typical number) have in this a 
mark of inferiority ; and thus the Phyllopods and Trilobites among Crustaceans 
bear marks of inferiority, the typical number of segments in the abdomen of a 
Crustacean being but seven, and in the whole body twenty-one,—each pair of 
members corresponding to one, commencing with the eyes as the anterior. 
Professor Agassiz has brought out and illustrated in his writings each of the 
above Criteria. 
(4.) Species having the largest number of distinct segments in the posterior part 
of the body, or having the body posteriorly prolonged, are the inferior among those 
under any type.—Shrimps and Lobsters are thus inferior to Crabs ; Centipedes, to 
Insects; Salamandrians, or tailed Batrachians, to the Frogs or tailless Batra- 
chians ; Snakes, to Lizards; the Ganoids with vertebrated tails, to those with 
non-vertebrated. It does not follow on this principle that Frogs, although tail- 
less, are superior to Lizards; for they are of different types of structure. 
(5.) Species having the anterior part of the body most compacted or condcnsed in 
arrangement, or having the largest part of the body contributing to the functions of 
the head-extremity, are the superior, other things being equal.Thus, Man stands 
at the head of all Vertebrates in having only the posterior limbs required for 
locomotion, the anlerior having higher uses ; and also in having the head most 
compacted in structure and brought into the least compass consistent with the 
amount of brain. In the same manner, the Carnivores among the large Mam- 
mals (Megasthenes) are superior to the Herbivores, the anterior limbs not having 
locomotion as their sole use, and the head being more compacted and con- 
densed for the size of brain. The highest Crabs, the Triangular or Maioids, are 
superior in the same manner to the lower, and far more to the Lobster tribe and 
other Macrourans ; descending in grade from the higher Crabs, the outer mouth- 
organs become more and more separated from the mouth, and finally, in many 
Macrourans, they have the form of feet, thus passing from the head-series to the 
foot-series. Insects are on this principle superior as a class to Crustaceans, 
although of so much less size. 
Condensation anteriorly and abbreviation posteriorly is the law of all pro- 
gress in embryonic development, and also of relative rank among species of 
related groups. 
Relation of the History of Life to the Physical History of the Globe 
1. The plan of progress was determined with reference to the last age, with 
all its diversities of climate, continental surfaces and oceans, us rts era of fullest 
exhibition. 
2. The progress in climate and other conditions mvolved a concurrent progress 
From the inferior living species to the superior.—The existence of a long marine 
era, through the Silurian and part of the Devonian ages, admitted only of the 
existence of marine life. Hence the dominant type of the Silurian was the 
Molluscan, which, with the Radiate, is eminently marine. In addition, there 
were marine Articulates and marine plants; and when the Vertebrates began 
it was with marine species, the Fishes. Thus the prevalence of waters involved 
inferiority of species. The increase of land, gradual purification of the atmose 
phere, and cvoling of the globe, prepared the way for the higher species, 
