The Geological Lesson. 103 



species. Although the horse in America was developed from a four 

 and five-toed animal no bigger than a fox to a single- toed animal the 

 size of an ass, it may be doubted whether this animal emigrated back to 

 Asia, and if he did not, the horse must have had in Asia a development 

 parallel to that in America. At any rate after the separation of the 

 continents and the changes in climate during the Quarternary, the horse 

 and camel became extinct in America but not in Asia. The same is 

 true of the lion, the elephant, the lemurs, the tapirs. 



Thus admitting that advanced life originated in some one spot, received 

 its most rapid development and diversification there and spread thence to 

 all other parts of the world, many of the phenomena admit of easy ex- 

 planation. Some of the early geologists observing that the first repre- 

 sentatives of a new type where first observed in Europe or America were 

 frequently not the lowest members of that type but that they foreshad- 

 owed forms inferior to themselves as well as superior, gave these repre- 

 sentatives the title of Comprehensive types. They appear to have im- 

 agined that they were introduced by supernatural power and placed on 

 exhibition as it were ; as a sort of sample average specimen of what 

 was to be expected next. At the same time they are constantly show- 

 ing how the oscillations of the land, the changes of continental outline 

 causing variations of climate and vegetal productions, also affect animal 

 life, causing changes of habit, action and food, and these reacting on 

 the organism cause gradual changes in its physical structure. They 

 show how it was impossible for land plants or animals to exist before 

 land, and how the marine species became extinct when the sea bottom 

 was elevated into dry land. Dana says there were 500 species of Trilo 

 bites, 900 species of the Ammonite group, 450 of the Nautilus, and 700 

 of Ganoid fishes, and of all animals nearly 40, 000 species have been 

 gathered from the rocks. * In many cases these species graduate into 

 each other in such a way that only an expert can separate them. Think 

 of 900 species in one family. All the above are extinct destroyed not 

 by supernatural interference but by adverse natural conditions. As we 

 can see the gradual development and general improvement of the species 

 in the strata accessible to us, we have no reason to doubt that similar 

 changes are recorded in the strata that have been sunk beneath the sea, 

 or buried below the impenetrable deposits of other continents. Prob- 

 ably during nearly half the time that elapsed between the close of the 

 Laurentian age and the beginning of the Tertiary our rocks were sub- 

 jected to a process of denudation, and the materials torn off were piled 

 away in the surrounding ocean. Such periods would be unfavorable for 

 the preservation of land animals or vegetation in situ, and if any were 

 preserved they are buried under the ocean, and marine remains would 



* Dana 601. More species are now known. 



