FIRST FORMS OF LIFE. 



37 



FIG. 14. 



| I,; 



trilobite, and the orth.ocerati.te (cephalopod) are everywhere 

 characteristic fossils. It has been remarked by Sir Roderick 

 Murchison, that in the Alleghany Moun- 

 tains, in the hills of Herefordshire, and 

 on the slopes of the Ural chain which 

 divides Europe from Asia, we have re- 

 mains of the same animal tribes con- 

 nected with this formation. There are 

 differences of species that is to say, the 

 fossils of different regions present certain 

 minor peculiarities, but even this is 

 only partial, and does not materially in- 

 terfere with the general fact that there 

 has been a remarkable uniformity of life 

 in the primeval seas. 1 In the present era, 

 it is hardly necessary to say, the case is 

 very different. Even seas so near as the 

 Red Sea and Mediterranean present wholly 

 different genera of mollusks. It has been 

 thought that there might be a cause for 

 the greater uniformity of life in those 

 ages, in the greater uniformity of tempe- 

 rature, resulting from the as yet unspent 

 heat of the surface, arising from the in- 

 ternal incandescence ; but perhaps the 

 more probable cause was simply the com- 

 parative newness of life upon earth, and 

 its little experience of those external agen- Orthoceratite. 

 cies by which it is liable to be affected, A, Exterior ; B, Sec- 

 and which, we shall see reason to }>Q- tion, showing the cham- 

 ,. , , , , ,1 i _jj oers and siphuncle. 



lieve, have conduced to the production 



of the many shades of variation which now mark the organic 

 kingdoms. 



[The uppermost Silurian rocks of the South of Scotland have 

 within the last three or four years (1860) presented us with 

 a number of crustaceans (Pterygotus, &c.) of large and small 

 size, including some which have been not less than six feet 

 long. Mr. Salter describes the pterygotus as " an elongated 

 crustacean, with a comparatively small head and sessile com- 



1 [Professor Edward Forbes considered this doctrine as losing force in 

 the progress of the science. He says (1854), " More and more evident 

 does it become every day that at all epochs, from the earliest preserved 

 to us to the latest, there were natural history provinces in geographical 

 space."] 



