610 DYNAMICAL GEOLOGY. 



water or in wet deposits are preserved from that complete decom- 

 position which many are liable to when exposed on the dry soil, and 

 are protected also from other sources of destruction. In North 

 America, during the Cretaceous period, the dry portions of the con- 

 tinent east of the Mississippi (see map, p. 489) were in all proba- 

 bility covered with vegetation as densely as now ; and yet we have 

 no remains of it, excepting the few in the Cretaceous beds of the 

 Atlantic and Grulf border. We may believe also that there were 

 numerous Mammals and birds in the forests, for Mammals began in 

 the Triassic, and birds in the Triassic or Jurassic, but not the first 

 specimen has anywhere been found. In the Pliocene Tertiary the 

 species of plants and birds may have been at least half as numerous 

 as now. Yet a few hundreds of the former and hardly a score of 

 the latter are all that have thus far been found fossil. The natural 

 inference from these facts is that, while we may conclude that we 

 have a fair representation in known fossils of the marine life of the 

 globe, we know very little of its terrestrial life, — enough to assure 

 us of its general character, but not enough for any estimates of the 

 number of living species over the land. 



Plants and all animal matter pass off in gases when exposed in the atmo- 

 sphere or in dry earth; and bones and shells become slowly removed in solution 

 when buried in sands through which waters may percolate. . Bones buried in 

 Wet deposits, especially of clay, are sealed from the atmosphere, and may re- 

 main with little change except a more or less complete loss of the animal por- 

 tion. Mastodons have been mired in marshes and thus have been preserved to 

 the present time ; while the thousands that died over the dry plains and hills 

 have left no relics. 



Among terrestrial Articulates, the species of insects that frequent marshy re- 

 gions, and especially those whose larves live in the water, are the most common 

 fossils, as the Neurojiters / while Spiders, and the insects that live about the 

 flowers of the land, are of rare occurrence. Waders, among Birds, are more 

 likely to become buried and preserved than those which frequent dry forests. 

 But, whatever their habits, birds are among the rarest of fossils, because they 

 usually die on the land, are sought for as food by numberless other species, and 

 have slender hollow bones that are easily destroyed. 



Vertebrate animals, as fishes, reptiles, etc., which fall to pieces when the animal 

 portion is removed, require speedy burial after death to escape destruction 

 from this source as well as from animals that would prey upon them. 



Fishes in the ocean, having the means of easy locomotion through the waters, 

 would be less liable to destruction from changes of level in the land than the 

 Mollusks of a coast; and hence some of the sharks of the Tertiary continue 

 through two or three epochs. 



The animals generally of the ocean are little liable to extermination from 

 changes of climate over the land; and hence some marine invertebrate species of 

 the Miocene Tertiary, many of the Pliocene, and all of the Post-tertiary, con- 



