22 GENERAL PRINCIPLES. 



3. The preceding list of species will show that all the great classes of animals and 

 plants have their representatives in nearly all the formations, though no vertebral animals 

 are found and no flowering plants in the lower silurian group. Indeed the plants and 

 animals lowest in organization are found in the oldest rocks, and the more perfect ones 

 have been gradually introduced, so as to make those now living the most perfect of all. 



4. The new races have been introduced not by the gradual change of one species into 

 another, but by the creation of new and successive groups. The earth has changed its 

 inhabitants entirely five or six times. Indeed some eminent writers reckon as many as 

 twenty seven life periods, each having its peculiar fauna and flora. 



5. Excepting a few hundred species in the alluvial and tertiary rocks, no living species 

 of animals or plants are found fossil. Below the tertiary every species has died out, 

 although the classes and families, and sometimes the genera, extend through the whole 

 series. 



6. The fossil species, even those in high latitudes, correspond more nearly with those 

 now living between the tropics than in the temperate or frigid zones ; proving that the 

 climate, when the fossil species lived, must have been tropical or even ultra-tropical. 



7. Not a few of the fossil species had forms quite peculiar and anomalous, exhibiting 

 characters now found only in different classes ; yet they were only peculiarities wisely 

 adapted to the condition of a changing world, and all belonged to that one great system 

 of organic beings, which embraces and links together all the minor systems of life both 

 living and fossil. 



* 



METAMORPHISM OF BOOKS. 



We suppose that every geologist considers it a settled principle, that all the rocks 

 accessible to man have undergone entire and most of them repeated changes of form and 

 character since their original creation. If the globe was once in a molten state, the crust 

 which first formed over its surface must have been some kind of unstratified rock. When 

 it became cool enough to allow water to condense on the surface and form oceans, the 

 waves would wear away portions of the rock and deposit the fragments in the form of 

 gravel, sand, and clay. These by the action of internal heat might be hardened and 

 become conglomerates, sandstones and shales. If new beds of materials should be thrown 

 down upon these strata, it would cause the internal heat to penetrate farther upward into 

 the conglomerates, sandstones and shales, and by the help of water render the rocks 

 plastic and convert them from mechanical into crystalline rocks, without destroying the 

 planes of stratification, though generally obliterating all traces of organic structures which 

 they might have contained, and changing the laminated structure into foliation and 

 cleavage. After all this, water may have acted mechanically on these strata, wearing them 

 away and forming other deposits of pudding stones, sandstones and shales. Meanwhile, 

 also, the internal heat, working farther upward, as it certainly would do by the accumula- 

 tion of new beds of detritus, might melt over the lower beds of the strata, converting them 

 into unstratified rock. And thus might the same materials have been subject to repeated 

 and most thorough metamorphosis. 



