598 HISTORICAL GEOLOGY. 



8. Comprehensive types generally becoming nearly or quite extinct in the 

 course of future progress. — See page 397 for illustrations. 



9. Unity in the successive Floras and Faunas of the ages. — The unity 

 or harmonious character of the Flora of the Carboniferous age, and 

 the dependence of this unity on the principle just explained, is the 

 subject of remark on page 396. It is a marked feature of each of 

 the ages. 



If the view of the Azoic age given on page 596 is right, this unity was 

 strikingly brought out in the first expression of life. In the Primordial life this 

 unity is equally marked. There were Brachiopods on stems, associated with the 

 unsymmetrical Cystids, also on stems, and more flower-like ; — and, with these 

 sedentary species, the Trilobite, nearly as sedentary in habit, — for it seems to 

 have clung to any supporting surface, like a limpet, though capable of swimming 

 off or crawling over the sea-bottom. The Gasteropods, Pteropods, and Phyllo- 

 pods were the more active species. A little later, before the close of the Pri- 

 mordial period, there were bivalve Crustaceans in harmony with the bivalve 

 Brachiopods. There was also a new type, indicating progress, in the large and 

 active Cephalopods, the Orthocerata, etc. 



The same general features continued to characterize the Lower and Upper 

 Silurian, only with additions to the flower-like animals in Corals, Crinoids, 

 and Bryozoans, and an increase in the diversity of Brachiopods and Trilo- 

 bites. The unity also appears in the simplicity of structure of the several types. 



In the Mesozoic Fauna there was also a wonderful harmony, as explained 

 on page 501. In the Mesozoic Flora there was a unity as striking as in 

 the Carboniferous. Conifers, Tree-ferns, and Cycads made up the bulk of 

 the trees, and the last type, while fundamentally related to the Conifers, 

 partook somewhat of the character of the Tree-fern in its mode of growth. 

 At the same time, this comprehensive type had some characteristics of the 

 palm, — the type it foreshadowed, and which, before the close of the Mesozoic, 

 was already in the forests along with the highest type of plants, the An-, 

 giosperms. 



10. Progress always an unfolding of a type or an exhibition of it in its 

 possible diversities, and involving the introduction of inferior as well as supe- 

 rior species. — -It has been already shown that the progress was not a 

 lineal upward progress. The facts with regard to comprehensive 

 types and the associated species throw this principle into a strong 

 light ; for these species occupy nodal points, as they may be called, 

 or points of divarication, far remote in most cases from the lowest 

 species of a group. 



The progress was not necessarily attended with much rise in 

 grade. The earliest fishes are of the highest orders in that class. 

 These orders undergo some little elevation in after-time ; but in the 

 introduction of the Teliosts, or common fishes, in which the great 

 expansion ultimately takes place, there is a fall below the level of 

 the early orders. 



