Vice- President' s Address. 191. 



evolved before Pliocene times. No one has seen this more 

 clearly than Mr Wallace/ with tlie general drift of whose 

 argument I am quite at one. In earlier ages, the warm water 

 of the tropics overflowed wide areas of our present continents 

 — most of the dry land was more or less insular, and the seas 

 within the Arctic Circle were certainly not cold as at present, 

 but temperate and even genial. If we go back to Cambro- 

 Silurian times, we find only the nuclei, as it were, of our 

 existing continents appearing above the surface of wide- 

 spread shallow seas. It is quite impossible, therefore, that 

 under such geographical conditions, great continuous ice- 

 sheets, like those of the Pleistocene, could have existed — no 

 matter how high the eccentricity of the earth's orbit may 

 have been. The most that could have happened during such 

 a period of eccentricity, would be the accumulation of snow- 

 fields on mountains and plateaux of sufficient height, the 

 formation here and there of local glaciers, and the descent of 

 these in some places to the sea. And what evidence of such 

 local glaciation might we now expect to find ? ISTo old land 

 surface of that far-distant period has come down to us : we 

 look in vain for Cambro-Silurian roches moutonnees and boulder- 

 clay or moraines. The only evidence we could expect is just 

 that which actually occurs, namely, erratics (some of them 

 measuring five feet and more in diameter) embedded in 

 marine deposits. It may be said that a few erratics are 

 hardly sufficient to prove that a true glacial period super- 

 vened in Cambro-Silurian times, and I do not insist that they 

 are. But I certainly maintain that if any lowering of the 

 temperature were induced by high eccentricity of the earth's 

 orbit during Cambro-Silurian times, then ice-floated erratics 

 are the only evidence of refrigeration that we need ever hope 

 to find. The geographical conditions of early Palaeozoic times 

 forbade the formation of enormous ice-sheets like those of the 

 Pleistocene period. Extreme climatic changes were then 

 impossible, and periods of high eccentricity might have come 

 and gone without inducing any modifications of flora and 

 fauna which we could now recognise. We are practically 

 ignorant of the terrestrial life of the globe at that distant 



^ See Island Life. 



