PROGRESS OF LIFE. 601 



tropical ; myrtles and laurels, in the sub-tropical ; evergreen trees, 

 in the warm-temperate ; ordinary deciduous trees, in the cold-tem- 

 perate ; and pines, in the sub-arctic. As there are now these zones 

 on going from the equator to the poles, so there were successive eras 

 passed over from the Silurian — the period of universal warm tempe- 

 rature — to the present age of a frigid arctic, and a mean temperature 

 of 58° to 60° JB\ Climate may not have been the only cause ; but it 

 was one, and of great importance. The Crustacean type is one of 

 those which have culminated in the age of Man ; and this accords 

 with the fact that its highest species — the Maioids, or Triangular 

 Crabs — are now most numerous and of the highest rank in the 

 colder temperate zone. It was made to reach its maximum in a 

 cold climate, and therefore in the existing age. 



No species survived through all time, and few through two suc- 

 cessive periods. The oldest now existing began in the Middle Ter- 

 tiary, and these were only Invertebrates. The oldest quadruped 

 dates fio farther back than the Post-tertiary. 



But two genera range through the whole series of ages from the 

 first or Potsdam epoch, — Lingula and Discina, — enough to manifest 

 the oneness of system from the beginning. There was in general a 

 changing of genera with the successive periods. Even tribes wholly 

 disappeared from age to age, as the world outgrew them. Of Trilo- 

 bites, 500 species once lived, of the Ammonite group, 900 species, 

 all of which are extinct ; the Nautilus tribe, 450 ; three or four spe- 

 cies are all that exist. Of Ganoid fishes, 700 species have been dis- 

 covered ; the tribe is now nearly extinct. Thus, the old has passed 

 away as the new has come in. Remains of nearly 40,000 animal 

 species have been gathered from the rocks, all of which are ex- 

 tinct ; and, considering how few of the whole number would have 

 become fossilized, this can hardly be one-tenth of the number that 

 have existed and are gone. 2500 extinct species of plants have 

 been found, — which cannot be over a twentieth of all that have 

 covered the earth in its former ages. 



4. The extermination of species was in general due to catastrophes, while 

 the extinction of tribes or higher groups may have been a consequence of secu- 

 lar changes in the condition of the climate, atmosphere, or waters. — The ex- 

 termination of species here alluded to, and some of the kinds of 

 catastrophes which caused them, are briefly considered on p. 398. 



5. With regard to the Origination of Species, Greology suggests no 

 theory of natural forces. It is right for science to search out Na- 

 ture's methods, and strive to employ her forces — organic or inor- 

 ganic — in the effort, vain though it prove, to derive thence new 

 lwing species. The study of fossils has given no aid in this direc- 



