PRESIDENTIAL ADDRESS. 633 



is with regard to the value of the evidence for interglacial periods ; and it will be 

 my aim, in bringing before you some general conclusions regarding the drifts, to 

 concentrate attention principally upon this evidence. 



To keep the discussion within practicable limits I must perforce assume the 

 former extension of ice-fields over the glaciated areas ; for although I know that 

 there are still dissentients from this fundamental proposition, the cumulative 

 evidence in its favour has been so frequently recapitulated that it would not be 

 justifiable for me to detain you by repeating the arguments. 



It is now, 1 think, agreed by all who accept this proposition that the ice- 

 sheets of the Glacial Period, though of vast extent, had their northern as well as 

 their southern limits ; the original idea, that they represented the outer portion of 

 a polar ice-cap, having been disproved by more extended researches in the more 

 northerly part of our hemisphere. Moreover, it has been found that these ice- 

 sheets had their origin in the coalescence of masses which spread outward 

 from separate areas of accumulation, acting more or less independently, so that 

 the individual sheets did not all attain their farthest bounds at the same time. 

 But this recognition of independent centres of glaciation has given sharper 

 prominence to the question whether the glacial deposits are to be regarded as the 

 product of a single epoch of glaciation, or whether they represent successive 

 epochs of this kind, separated by intervals during which the great ice-sheets 

 temporarily vanished. 



As opinion stands at present, probably most geologists lean to the idea that the 

 glaciation was interrupted by at laast one interglacial epoch, during which the 

 climate of any particular latitude became not less warm, and perhaps warmer, 

 than it now is. This is the Interglacial hypothesis in its simplest form. But 

 it has been frequently pointed out that the criteria depended upon in the 

 recognition of warm interglacial conditions cannot be all assigned to the 

 same horizon, since they recur at different positions in the drift series. Hence it 

 has been claimed that two, three, four, or even five interglacial epochs, with a 

 corresponding number of separate epochs of glaciation, may be recognised in the 

 glacial sequence. In respect to the number, relative importance, and correlation 

 of these epochs or stages in different countries, or in different parts of the same 

 coimtry, there has been, however, no pretence to agreement among the upholders 

 of the Interglacial idea. 



In opposition to these views of every degree, a smaller number of glacialists 

 have urged that there is no proof of even a single absolute interruption of the 

 glacial conditions from the beginning to the end of the period ; and that the 

 evidence indicates only one great glaciation, during which there were wide oscilla- 

 tions of the margins of the ice-sheets in diflerent places, due probably to more or 

 less local circumstances. 



This radical difference of interpretation respecting the constitution of the 

 Glacial Period assumes the greater consequence in that it bears directly upon 

 many questions other than those which are strictly geological. Thus, the ante- 

 cedents and distribution of our present fauna and flora, and the time and con- 

 ditions of that momentous event, the appearance of man in Northern Europe, are 

 deeply involved in the issue. 



Moreover, until we can tell whether it is one or several periods of glaciation 

 that we require, how can we approach the other sciences for aid in our search for 

 the cause of the Ice Age ? It is, indeed, essential that, before seeking counsel's 

 opinion of this kind, the geologist should have all his evidence at command and 

 well-marshalled, so that he can say such and such are the facts, and this the 

 order of them. Otherwise he may receive, not the desired interpretation, but 

 advice as to what he ought to have found and instructions to go and find it. And 

 that such instructions may be detrimental rather than helpful to our progress is, 

 I think, shown by the history of the Interglacial hypothesis. In this matter the 

 glacial geologists, having some evidence for the alternate extension and recession 

 of ancient glaciers, fell readily under the influence of the fascinating theory brought 

 forward by James Croll to explain the Great Ice Age, whose interpretation, how- 

 ever, reached far beyond the facts thH.t were placed before him. 



