52 REVIEWS—A MANUAL OF GEOLOGY. 
It is probable that the oceanic waters were also in an impure state compared 
with the present, from containing an excess of salts of lime; and, this also 
involved the existing of inferior species,—such as Crinoids, Corals, and Mol- 
lusks, a very large proportion of whose weight is in calcareous material, The 
removal of this excess of lime from the waters produced limestone strata). 
purified the waters, and fitted the oceans for other species, 
The great prevalence, in the Primordial, of Lingule (whose shells contain a 
large amount of phosphate of lime) is further evidence of the greater density 
of the waters, and seems to indicate the presence of an excess of phosphates. 
3. The progress in climate and in the condition of the atmosphere and waters 
involved a localization of tribes in time, or chronographically, just as they are 
now localized by climate over the earth’s surface, or geographically —Tribes were 
made for a special climate or condition of the globe; and when this climate or 
condition had been passed in the earth’s progress, the tribes no longer existed. 
The culmination of the Reptilian and Molluscan types in the Reptilian age, or of 
Trilobites and Brachiopods in Paleozoic time, are examples. The former when 
instituted had those special relations to climate that made the Reptilian age the 
era of their culmination; just as now palms and bananas reach their perfection 
only in the equatorial zone; figs in the tropical; myrtles and laurels, in the 
sub-tropical ; evergreen trees, in the warm-temperate ; ordinary deciduous trees, 
in the cold-temperate; and pines,,in the sub-arctic. As there are now these 
zones on going from the equator to the poles, so there were successive eras 
passed over from the Silurian—the period of universal warm temperature—to 
the present age of a frigid arctic, and a mean temperature of 58°-to 60° F. 
Climate may not have been the only cause; but it was one, and of great impor- 
tance. The Crustacean type is one of those which have culminated in the age 
of Man; and this accords with the fact that its highest species—the Maioids, or 
Triangular Crabs—are now most numerous and of the highest rank in the 
colder temperate zone. It was made to reach its maximum in a cold climate,. 
and therefore in the existing age. 
No species survived through all time, and few through two successive periods; 
The oldest now existing began in the Middle Tertiary, and these were only 
Invertebrates. The oldest quadruped dates no farther back than the Post-- 
tertiary. 
But. two genera range through the whole series of ages from the first or 
Potsdam epoch,—Zingula and Discina,—enough to manifest the oneness of 
system from the beginning. There was in general a changing of genera with 
the successive periods. Even éribes wholly disappeared from age to age, as the 
world outgrew them. Of Trilobites, 500 species once lived, of the Ammonite 
group, 900 species, all of which are extinct ; the Nautilus tribe, 450; three or 
four species are all that exist. Of Ganoid fishes, 700 species have been dis- 
covered; the tribe is now nearly extinct. Thus, the old has passed away as the 
new has come in. Remains of nearly 40,000 animal species have been gathered 
from the rocks, all of which are extinct ; and, considering how few of the whole 
number have become fossilized, this can hardly be one-tenth of the number that. 
have existed and are gone. 2,500 extinct species of plants have been found,—- 
