part 2] ANNIVERSARY ADDRESS OF THE PRESIDENT. lxxvii 



origin must be assigned to the genera described by Walcott. The 

 striking resemblance between the Archaean and the Permian struc- 

 tures has been recognized independently by Prof. Holtedahl, who 

 published some photographs of Magnesian Limestone concretions 

 indistinguishable in any essential features from illustrations pub- 

 lished by Walcott. I fail to see that the American specimens 

 have any claim to be regarded as alga?, and I venture to think 

 that they afford no real evidence of the co-operation of plants in 

 their formation. There can be no doubt that chemical and physical 

 factors are sufficient in themselves to produce various types of 

 structure which are often attributed to the agency of plants : it 

 is not unreasonable to suggest that such specimens as Dr. "Walcott 

 has described owe their peculiar features to inorganic, and not to 

 organic, causes. Experiments made some years ago by R. E. 

 Liesegang demonstrated the important bearing of diffusion pheno- 

 mena upon the general question of rock-structure. He found that 

 if a coagulated colloidal solution, a gel, contains a substance in 

 solution, and a second solvent capable of reacting with the former 

 is allowed to diffuse into it, reaction takes place, but not con- 

 tinuously; with the result that the product is deposited in strata 

 separated by apparently clear intervals. If a solution of sodium 

 carbonate is added to a test-tube partly filled with 1 per cent, 

 agar-gelatine containing calcium chloride, calcium carbonate is. 

 deposited in' a succession of strata. It is at least worth con- 

 sidering whether there may not be more than a superficial resem- 

 blance, between the structures produced experimentally and various 

 Stromatoliths and similar bodies generally regarded as organic, and 

 frequently described as algae. 



My primary object in attempting briefly to summarize the 

 various kinds of evidence which have been brought forward in 

 reference to life on Archaean land and in Archaean seas is to 

 emphasize the importance of submitting such documents as are 

 available to a searching examination. It is tempting to endow our 

 own remote ancestors with qualities that we would wish them 

 to have possessed: similarly,. a student of evolution in the wider 

 sense may easily allow to fancy more scope than the facts warrant. 

 We know nothing of Archaean land- vegetation. Negative evidence 

 lends support to the suspicion that the continents were bare of 

 plants. The oceans may have been inhabited by hosts of uni- 

 cellular alga 1 , and higher forms may have lived on the shelving 

 sea-margins; but of the actual nature of these plants we have no 

 certain knowledge. 



