Tlic Lazi'S of Climatic Evoliitioji, — Manson. 45 
which require an Ice age, single in its occurrence and unique 
in the cHmatic history of the earth. The theories of this class 
proved unsatisfactory, and failed to fully explain the admitted 
facts of geology. This failure, in a measure, warranted a re- 
sort to the ingenious theories of Class II ; which theories re- 
(juire a recurrence of glacial epochs. The lack of evidence of 
such recurrence has prevented the theories of this class from 
being generally accepted. 
Class I. — (i) A decrease in the original heat of the earth. 
(2) Changes in the elevation of the land areas, and con- 
sequent variations in the distribution of land and water. 
(3) A period of greater moisture in the atmosphere. 
Class II. — (4) Changes in the obliquity of the axis of the 
earth. 
(5) A coincidence of an aphelion winter with a period of 
maximum eccentricity of the orbit of the earth. 
(6) A combination of (2) and (5). 
These have been frequently reviewed by various author- 
ities. The general conclusions reached may be summed up 
in the following opinions: 
Referring particularly to the first cause of the series, and 
probably the oldest. Prof. Jas. D. Whitney says: 
"It is evident that the idea of connecting the phenomena of the in- 
ternal heat of the globe with terrestrial climates, whether of the pres- 
ent or of past geological ages, must be entirely abandoned, as it has 
been by most writers on this subject. The hypothesis can not be al- 
lowed to stand as even one of the possible theories of climatic change." 
(The Climatic Changes of Later Geological Times, page 261. See also 
LeConte, Elements of Geology, 3rd Ed., page 381. Shaler and Davis. 
Glaciers, page 70.) 
After an exhaustive study and review of all the principal 
theories, Dr. James Geikie says: 
"The primary cause of these remarkable changes is an extremely 
perplexing question, and it must be confessed that a complete solu- 
tion of the problem has not been found. Croll's Theory has un- 
doubtedly thrown a flood of light upon our difficulties and it may be 
that some modifications of his views will eventually clear up the mys- 
tery. But for the present, we must be content to work and wait." 
(The Great Ice Age, 3rd Edition, 1894. page 816.) 
After reviewing the principal theories as to the cause of 
the Ice age, Dr. Jos. LeConte remarks of Dr. Wallace's mod- 
