60 THE LESSON OF EVOLUTION 



belong to a group which at the present day floats 

 near the surface of the sea. This pelagic aspect of 

 the early faunas is carried out by the Mollusca of the 

 Algonkian and Cambrian periods, as well as by the 

 great development of free-swimming Medusae in the 

 Cambrian ; and we should remember that these delicate 

 pelagic animals must have been very numerous to 

 have left any record at all. 



The earliest known Kadiolarians are accompanied 

 by the remains of sponges which must have lived on 

 the bottom of the ocean, and these were followed by 

 creeping worms and Trilobites. The early Brachiopods 

 have diaphanous shells, like the pelagic mollusca ; but 

 it seems probable, from a study of their development 

 in living forms, that at first they had no shell at all, 

 but consisted of the peduncle encased in a sand-tube. 

 The shell was afterwards added to protect the branchiae, 

 and in course of time the intestinal tract in the 

 peduncle atrophied. Perhaps the so-called annelid 

 tubes of the Torridon sandstone represent the first 

 Brachiopods. 



From all this we may infer that the first animals 

 were pelagic protozoa, which in time varied and gave 

 rise to pelagic worms and mollusca. At a very early 

 date, however, some of the protozoa followed down 

 the dead organisms and settled on the bottom, giving 

 rise to the sponges. Afterwards worms moved in the 

 same direction, feeding, probably, on the sponges ; 

 and from them are descended the Brachiopods and the 

 Crustaceans. 1 



1 This theory was originated, I think, by Biologists, and was first 

 brought prominently before Geologists by Professor W. K. Brooks, in 

 the American Journal of Geology for July and August, 1894. 



