394 NOTES OF A NATURALIST, 



The latter author holds, in common with Sir Charles Lyell, that 

 geographical causes, arising from the varying distribution of 

 land and sea, have mainly controlled the distribution of tempera- 

 ture over the earth's surface ; but he is disposed to go farther 

 than Lyell in admitting the influence of periods of high eccen- 

 tricity in causing those great accumulations of snow and ice 

 which were requisite to produce the phenomena of a glacial 

 period, whenever a sufficient area of elevated land in high 

 latitudes coincided with the period of high eccentricity. 



It would probably be of little avail, even if I were to under- 

 take the task, that I should attempt any thorough discussion of 

 this vast and difficult problem ; and it would certainly require 

 far more space than can here be given to it. I may, however, 

 venture to make a few remarks upon some points which have 

 not, to the best of my knowledge, been much noticed in the 

 discussion. 



In reading Mr. Croll's work, which charmed many an hour 

 during the voyage to and from South America, I found it very 

 difficult to discover any flaw in the chain of close reasoning by 

 which he supports his conclusions. Most of the facts on which 

 he relies are warranted by observation, and have been accepted 

 as well established by writers of the highest authority ; and his 

 inferences as to the results of altered conditions appeared to be 

 in strict conformity with admitted physical principles. Never- 

 theless, when I reflected on the anomalies which are found at 

 the present time in respect to the climate of many spots in the 

 world, and the complexity of the causes which determine its 

 actual condition, I felt a doubt whether, in his attempt to trace 

 the result of possible changes, Mr. Croll may not have over- 

 looked some of the elements of the problem. 



Let me briefly state the leading propositions of Mr. Croll's 

 theory in order to make intelligible the succeeding remarks. 



Estimating approximately the mean distance of the earth 

 from the sun at ninety-one and a half millions of miles, and the 

 eccentricity * of the sun's place in the orbit at one and a half 

 million, it follows that at one period of the year, which happens 

 to be about the winter solstice of the northern hemisphere, the 

 earth receives from the sun a quantity of heat greater than that 



* I use the term "eccentricity" in the popular sense, to express the 

 distance of the focus from the centre of the ellipse. 



