64 THE LESSON OF EVOLUTION 



are the Graptolites and the Brachiopods. The Grap- 

 tolites are known in North America from the Lower 

 Cambrian to the Carboniferous ; but in Europe they 

 first appear in the Upper Cambrian as a monoprio- 

 nidian form allied to Dichograptus. In the lowest beds 

 of the Ordovician they suddenly attain their greatest 

 development, after which they gradually declined, 

 and only a few forms pass into the Devonian. The 

 earlier forms had many irregular branches, which 

 during the Ordovician period decreased in number, 

 and became regularly arranged ; while throughout the 

 greater part of the Silurian we find only simple, un- 

 branched forms. The thecre also, which were at first 

 straight and with straight apertures, became curved 

 and with curved apertures often produced into a spine, 

 and in the Silurian period the aperture was in some 

 cases still more complex. The species of Graptolites 

 are widely spread geographically, and occur in very 

 dissimilar rocks, such as limestones, shales, and grits. 

 Sometimes they are accompanied by a varied fauna ; 

 but in other places they occur in thin zones without 

 any other fossils ; while the different species which 

 characterise these zones are the same, and have the 

 same vertical distribution, wherever they are found. 

 The explanation of these facts appears to be that the 

 Graptolites floated on the surface, and consequently 

 were independent of the depth of the sea and the 

 nature of the sea bottom. We find additional evidence 

 in confirmation of this in the fact that some of the 

 early species were furnished with a disc, which probably 

 acted as a float. 



The Brachiopoda show a remarkable development 

 during both the periods under consideration. Orthis, 



