28 THE AGE OF THE EARTH 



the purpose of this address I am considering as the 

 lowest fossiliferous stratum. We are looking forward 

 to the new light which will be thrown upon this form in 

 the communication of its veteran defender, Sir William 

 Dawson, whom we are all glad to welcome. 



Passing by the Radiolaria, with delicate skeletons less 

 suited for fossilization, and largely pelagic and therefore 

 less likely to reach the strata laid down along the fringes 

 of the continental areas, the next Phylum which is found 

 in a fossil state is that of the Porifera, including the 

 sponges, and divided into two Classes, the Calcispongiae 

 and Silicospongiae. Although the fossilization of sponges 

 is in many cases very incomplete, distinctly recognizable 

 traces can be made out in a large number of strata. 

 From these we know that representatives of all the 

 groups of both Classes (except the Halisarcidae, which 

 have no hard parts) occurred in the Silurian, Devonian, 

 and Carboniferous systems. The whole Phylum is an 

 example of long persistence with extremely little change. 

 And the same is true of the Nematophora : new groups 

 indeed come in, sometimes extremely rich in species, such 

 as the Palaeozoic Rugose corals and Graptolites ; but 

 they existed side by side with representatives of existing 

 groups, and they are not in themselves primitive or an- 

 cestral. A study of the immensely numerous fossil corals 

 reveals no advance in organization, while researches into 

 the structure of existing Alcyonaria and Hydrocorallina 

 have led to the interpretation of certain Palaeozoic forms 

 which were previously obscure, and the conclusion that 

 they find their place close beside the living species. 



All available evidence points to the extreme slowness 

 of progressive evolutionary changes in the Coelenterate 

 Phyla, although the Protozoa, if we may judge by the 

 Reticularia (Foraminifera), are even more conservative. 



When we consider later on the five Coelomate Phyla 

 which occur fossil, we shall find that the progressive 

 changes were slower and indeed hardly appreciable in 

 the two lower and less complex Phyla, viz., the Echino- 

 derma, and Gephyrea, as compared with the Mollusca, 

 Appendiculata, and Vertebrata. 



