594 GEOLOGICAL HISTORY. 



existed. The culmination of the Reptilian and Molluscan types in 

 the Reptilian age, and of Trilobites and Brachiopods in Paleozoic time, 

 are examples. The former, when instituted, had those special relations 

 to climate that made the Reptilian age the era of their culmination ; 

 just as, now, Palms and Bananas reach their perfection only in the 

 equatorial zone ; Figs, in the tropical ; Myrtles and Laurels, in the 

 subtropical ; and Pines, in the subarctic. As there are now different 

 zones of living species on going from the equator to the poles, so there 

 were successive phases in the life of the world passed over from the 

 Silurian — the period of universal temperate climate — to the present 

 age of a frigid Arctic, and a mean temperature of 58° to 60° F. Cli- 

 mate was not the only cause ; but it was one, and of great import- 

 ance. 



4. The progress was in accordance with system. — The species fol- 

 lowed one another, according to a system of mutual relation or depen- 

 dence, which is so profound and comprehensive that this progress is 

 rightly spoken of as an evolution or development. This statement 

 is sustained by the following considerations : — 



(1.) The same grander types of structure that appeared in the Si- 

 lurian age continued to be the grander types through all subsequent 

 time. The Vertebrate type, for example, which was represented before 

 the Silurian age closed, presented in its early species the fundamental 

 elements of all Vertebrates ; and future progress was manifested 

 in modifications and complete developments of the fundamental idea. 

 The two pairs of fins in Fishes represent the two pairs of limbs of 

 higher species ; an air-bladder, the lungs ; a loose-bone in a closed 

 cavity, the ear ; and so on throughout the structure ; and this is so 

 completely true that the comparative anatomist, in order to understand 

 the skeleton of the Mammal, or of Man, goes to the Fish for instruc- 

 tion. Thus the whole animal kingdom is the display of a few com- 

 prehensive structural types — the simpler forms of which appeared 

 in early time, and the more complex came forth successively afterward. 

 Some new organs were required in the highest manifestations of a 

 type. But these were only developments through modification of 

 the older, or better appliances evolved from the structure for carrying 

 forward old processes. 



Further, some of the old Silurian families of Invertebrates con- 

 tinued to exist through all time to the present. Thus, the most ancient 

 type of Mollusks yet discovered, the Lingula family, is represented by 

 species in our present seas ; and so also the Discina and Nautilus fami- 

 lies. Among Vertebrates, some of the ancient Gars are very much 

 like our modern kinds, and one Triassic genus, Ceratodus, is still repre- 

 resented in Australian Seas. Such facts, coming up from the past, 



