400 NOTES OF A NATURALIST. 



The differences between the mean results given above are 



shown by the following table, in which the signs show the 



excess or deficiency of the southern as compared with the 

 northern hemisphere : — 



Warmest month. Coldest month. Annual mean. 



- iri° Fahr. + i8-i° Fahr. + 4-2° Fahr. 



Dr. Hann has carefully discussed the question as to the com- 

 parative mean temperatures of the two hemispheres in a paper 

 pubhshed in the proceedings of the Vienna Academy, the 

 substance of which is given in his Kltinatologze, pp. 89, et seq.j 

 and it is difficult to refuse assent to his conclusion that so far 

 as the available evidence goes, it shows that the mean tempera- 

 ture of both hemispheres is equal. 



I find, then, that the same train of reasoning by which Mr. 

 Croll has sought to explain the occurrence of glacial periods by 

 changes in the eccentricity of the earth's orbit, and the pre- 

 cession of the equinoxes, leads us to conclusions respecting the 

 climatal condition of the different parts of the earth, at the 

 present amount of eccentricity, which are altogether opposed to 

 the results of observation ; and I am driven to the conclusion 

 that the causes which he has adduced have not the predominant 

 influence which he has attributed to them, and that there must 

 be other agencies to which he has not assigned their due 

 importance, but which are adequate to counteract the efficiency 

 of those which, as observation proves, fail to achieve the effects 

 anticipated from them. 



I am far from pretending to be able to analyze completely the 

 complex agencies which, by their mutual action, determine the 

 climate of different parts of the earth, but I may briefly refer to 

 two of them. Foremost of these is the relative distribution of 

 land and sea, for a due appreciation of which we are indebted 

 to the great work of Sir Charles Lyell. It is unnecessary here 

 to discuss how far his view of the probable amount of change 

 in past geological epochs may, in the present state of our 

 knowledge, be subject to limitation. Mr. Wallace, who is the 

 most strenuous supporter of the modern doctrine of the per- 

 manence of the present continents and ocean basins, recognizes 

 the theoretical correctness of Lyell's views, and admits that 

 changes of level great enough to cause profound modifications 



