T. C. Chamberlin — Diversity of the Glacial Period. 175 



hypotheses. That method requires that all hypotheses not 

 absolutely excluded by evidence should be retained in the 

 working group. If any doctrine that can properly be desig- 

 nated a doctrine of unity shall ultimately prevail, it will be 

 one that will be developed along the lines of effort which 

 characterize this class and distinguish it from the first class. 



So far as the attitude of those who entertain this class of 

 views concerns analytical methods of working, it stands, so far 

 as it is typical, in sharp antagonism to that of the advocates of 

 the first class, because the ultimate establishment of their 

 views depends upon the clear discrimination of the glacial 

 deposits that were formed in the apparent intervals appealed to 

 by the advocates of the third and fourth classes and upon 

 the demonstration that such episodes of glaciation occupied 

 these intervals. This involves not only the clear discrimina- 

 tion of the intervals but the determination of the special 

 deposits that bridge them. This demands the utmost re- 

 sources and highest refinements of analytical and discrimi- 

 native methods. Success will not He in ignoring or belittling 

 the extension of glacial time or the greatness of the inter- 

 vals now discovered ; but, if attained at all, it will be through 

 ability to fill in the gaps with undiscovered deposits and 

 to bridge over the unquestionable changes of surface attitude 

 with gradational stages and concurrent glaciation, and to 

 demonstrate that the variations in the character of glacial action 

 took place by such gradual steps as to bind the whole into 

 inseparable unity. This requires an attitude toward discrimi- 

 native work precisely opposite that assumed in Professor 

 Wright's discussion of the subject. 



3. It is difficult to name, and moreover unprofitable to 

 attempt to name, the glacialists who at the present hour advo- 

 cate the doctrine of duality as distinguished from plurality or 

 diversity, since so many of those who some time ago felt com- 

 pelled by evidence to believe in at least one interglacial epoch, 

 but who were not convinced that it was necessary to recognize 

 more than one of the major type, have in recent years passed 

 on to, or at least toward, the recognition of two or more such 

 intervals under the impress of the accumulation of evidence. 

 No one can have followed conscientiously the onward move- 

 ment of opinion during the last few years and have failed to 

 note the strong drift from simple duality toward either a 

 strongly diversified duality or toward plurality. In the midst 

 of this movement, it is idle, as well as harmful, to attempt to 

 define the positions of individual glacialists in so far as they 

 have not recently defined them for themselves. 



4. What has just been said applies to the lower limit of the 

 fourth class, but there are some whose declared opinions make 



