THE PROGRESS OF LIFE 59 



branchs still inherit the twist in the visceral nerve- 

 loop which is characteristic of the Prosobranchs. 

 The early Pelecypoda were very minute, none of them 

 being more than a quarter of an inch in length. The 

 shells of all the Mollusca consist chiefly of a horny 

 substance, containing but a small quantity of phosphate 

 of lime, and much less of carbonate of lime ; thus 

 differing from the later shells, which are composed 

 almost entirely of calcic carbonate. 



No satisfactory evidence of plant life has been 

 found in any of the Cambrian rocks, unless Oldhamia 

 and the Lower Cambrian oolitic limestones of South 

 Australia 1 are proofs of the existence of calcareous 

 algae. Eophyton, which was formerly thought to be a 

 plant, has been shown to be due to the trailing of the 

 oral lobes of a Medusa over soft mud. 



SPECULATIONS ON PRE-ORDOVICIAN LIFE 



The first life was pelagic. Let us now see what 

 these dry facts teach us. In the first place, it is very 

 remarkable that the only extensive pre - Cambrian 

 fauna is composed of Eadiolarians and Sponges ; and, 

 as the Sponges are more complex animals than the 

 Eadiolarians, we must suppose that they are descended 

 from them, and not the Radiolarians from the 

 Sponges ; consequently the Radiolarians are the oldest 

 organisms we .know. No doubt the Radiolarians of 

 Brittany are not the first of their class ; nevertheless 

 we seem to have got as near, perhaps, as we ever can 

 get to the first organisms ; and we find that they 

 1 Pro. Linn. Soc. of N. S. Wales, vol. xxi. pp. 571 and 574 (1897). 



