3o6 Oil lii ilish Dn'fls ami llic Iiitcrolncld! Prob'cm. 



In opposition to these \iews of every dei,^ree, ;i smaller 

 number of i^lacialists have ur^ed that there is no proof of even 

 a single absohite interruption of the <4lacial conditions from tlie 

 bei^innino- to the end of the period ; and that the evidence 

 indicates onI\- one great g^Iaciation, during- whicli there were 

 wide oscillations of the marg^ins of the ice-sheets in different 

 places, due probably to more or less local circumstances. 



This radical difference of interpretation respecting- the 

 constitution of the Glacial Period assumes the g-reater conse- 

 quence in that it bears directly upon many questions other than 

 those which are strictly g-eolog-ical. Thus, the antecedents and 

 distribution of our present fauna and flora, and the time and 

 conditions 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 g-laciation that we require, how can we approach the 

 other sciences for aid in our search for the cause of the Ice 

 Ag-e ? It is, indeed, essential that, before seeking; counsel's 

 opinion of this kind, the g-eolog-ist should ha\e all his e\idence 

 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 oug-ht to have found and instructions to g-o and fuid it. 

 And that such instructions may be detrimental rather than 

 helpful to our prog-ress is, I think, shown by the history of the 

 Interg-lacial hypothesis. In this matter the g-lacial g-eolog-ists, 

 having- some evidence for the alternate extension and recession 

 of ancient glaciers, fell readily under the influence of the 

 fascinating- theory broug-ht forward by James Croll to explain 

 the Great Ice Age, whose interpretation, however, reached far 

 beyond the facts that were placed before him. 



I need hardly remind you that, according- to Croll, a suflicient 

 explanatioii of the Glacial Period could be found in certain 

 astronomical conditions, which were shown by his calculations 

 to ha\e recurred at definite intervals, and wore supposed to 

 have produced repeated alternations of cold and warm climate 

 at the opposite hemispheres during the course of the period. 

 It is not my purpose to discuss this or any other theory 

 regarding the cause of the Great Ice Age, but only to direct 

 your attention to tlie influence of CroH's views upon the work 

 of observation. If the theory could have been sustained, it 

 would have given into the hands of the geologist a first instal- 

 ment of that absolute nieasure of geological time which he so 



Naturalist, 



