592 HISTORICAL GEOLOGY. 



the whole calculation from its foundation, and renders it almost, if not quite, 

 worthless. An estimate of the length of the Miocene epoch made from data 

 derived from observations on the deposits then forming in England would have 

 given no idea of the length of time required for the Miocene molasse of Switzer- 

 land; and, in the same manner, any such data from observations at the present 

 day must be equally fallacious. When the estimate, as from delta deposits, is 

 based on the amount of detritus discharged by a stream, it is of more value. 

 But even here there is a source of great doubt, in our ignorance of the oscillations 

 the continent may have undergone in past time, which, especially if an upward 

 movement, would have affected the amount of discharge. This source of doubt 

 affects also the calculations from the excavation of valleys. 



2. GEOGRAPHICAL PROGRESS IN NORTH AMERICA. 



The system of oscillations and progress during the ages to the 

 close of the Tertiary period, and the new system which succeeded 

 and characterized subsequent time, have been discussed in the 

 course of the General Observations on the Azoic, Palaeozoic, Meso- 

 zoic, and Cenozoic eras ; and the reader is here referred to pp. 144, 

 388, 502, and 568, a recapitulation in this place being unnecessary. 



3. PROGRESS OF LIFE. 



Several general principles connected with the progress of life 

 have been illustrated in the course of the preceding history. They 

 are here brought together and presented briefly and more system- 

 atically. The subject may be considered under two heads -.—first, 

 the system in the progress of life ; second, the relations of the pro- 

 gress of life to the physical progress of the globe. 



1. System in the Progress of Life.* 



1. Reality of the progress. — (1.) In geological history, Mollusks, Co- 

 rals, and Crinoids are at one end of the series of animal life, Man 



* The following are some of the Criteria of rank among Animals : — 

 (1.) Under any type, water-species 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 inferior 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 vertebras, like Fishes ; 

 the Marsupials and Edentates, inferiority to other Mammals in having the 

 sacrum consisting of only two united vertebras, as in most Reptiles. On the 

 contrary, the Dinosaurs show their superiority to other Saurians in having the 

 sacrum made of five (or six) vertebras, as in the higher Mammals. 



