LIFE. 613 



The Excrements of animals afford a considerable amount of phosphates, and, by de- 

 composition, amnioniacal compounds, as in the case of guano. The amount of phos- 

 phates, from the life which swarms in some muddy sea- bottoms and shores, must be 

 large. For analyses of Coprolites, see page 61. 



Bones are combined with so large an amount of animal gelatine that they are the 

 food of various animals; and this is a great source of their destruction. Again, when 

 the animal matter decays, the bones are left very fragile, unless hardened anew by a 

 substitution of mineral matter. In the Cartilaginous fishes, the backbone, when it fails 

 wholly of stony material, is not found fossil, as in most fossil Ganoids. 



The teeth of Vertebrates contain much less animal matter than bones, and also a 

 coating of enamel, in which there is considerable phosphate of lime. They are there- 

 fore exceedingly durable, and the most abundant of the remains of many species. The 

 bony enamelled scales of Ganoid fishes are also phosphatic, and equally enduring, differ- 

 ing much in this respect from the membranous scales of Teliosts. 



(b.) The fitness of species for becoming fossilized or concreted into 

 rocks depends in part on their place and habits of groioth. 



Aquatic species of plants and animals are those most likely to be- 

 come fossils, and so to contribute to rock-formations ; and, next, those 

 that live in marshes, or along shores or the borders of marshes. The 

 reasons are two : (1) Because almost all fossiliferous rocks are of 

 aqueous or marsh origin ; and (2) because organisms buried under 

 water, or in wet deposits, are preserved from that complete decompo- 

 sition 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 continent, east 

 of the Mississippi (see map, p. 479), were in all probability 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 Gulf 

 borders. In the Pliocene Tertiary, the species of plants and birds 

 may have been at least half as numerous as now. Yet a few hun- 

 dreds 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. 



Plant 5 and all animal matter pass off in gases, when exposed in the atmosphere 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 remain with little change, except a more 

 or less complete loss of the animal portion. 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 regions, 

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

 Neuropters ; 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 pre- 



