BIOLOGICAL CHANGE IN GEOLOGICAL TIME. 105 



the groves of algcie, with the trilobites and molluscs peopling those 

 ancient seas, while along their coasts volcanic fires at intervals 

 break forth, and lavas are outpoured that cover the surrounding 

 rocks with basaltic or trachytic coatings. But save for these 

 volcanic outbursts, tlie crash of thunder, and the roar of wind 

 and wave, a silent world it was. No lowing herds or roaring beasts 

 of prey were on the land, and no birds sang their songs either 

 on tree-top or high upon the wing. And how desolate was the 

 unnavigated sea, for whales and porpoises, seals and sharks, and 

 flying fishes were not in its waters and no sea bird's mew was 

 heard, for no stormy petrel, gull or penguin was upon its surface. 



Change and Environment. 



If the conclusion is warranted that the cosmic inorganic 

 conditions on the globe, however locally or even regionally 

 varied, have been generally similar during the whole period of 

 the deposition of the sedimentary rocks, and therefore during 

 the whole period of the life on the globe that has given all the 

 information we possess of biological change, we must, I think, 

 further conclude that this change has accompanied in its progress 

 small rather than great alternations of environing conditions. 

 It is also evident from the testimony of the rocks that while 

 great biological changes have synchronised with very small, if 

 any changes of environment, slight biological changes and even 

 morphological continuance, have accompanied considerable 

 alterations of environing conditions. 



The marine conditions of the Ludlow could have been little 

 different from those of the Wenlock period, during both of 

 which argillaceous and calcareous matter was largely deposited, 

 giving the Ludlow and Wenlock shales and limestones, and yet 

 the fauna of the one gives us Yertebrata which is absent in the 

 other ; corals and echinoderms greatly decreased ; other 

 Invertebrata greatly alter ; and large arachnid Crustaceans take 

 the place of many species of trilobites. The British Permian 

 deposits of sandstones and marls show similar marine conditions 

 to those indicated by the Triassic sandstones and marls, lied 

 sandstones with conglomerates and stiff red marls make up 

 3,000 feet of the Permians of this country, and red and 

 variegated sandstones with conglomerates and stiff red marls 

 make up 3,000 feet of the- Trias of England. Yet our Permian 

 mollusca is wanting in our Triassic rocks, while the homocercle 

 fishes and the dinosaurs of the Trias are altogether wantinii in 

 the Permian. Indeed the Triassic rocks are much more allied 



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