272 NOTES OF A NATURALIST. 



markable ability and perseverance with which he has 

 sustained his views, geologists and students of every 

 other branch of natural science have learned to 

 estimate the influence which the secular changes in 

 the eccentricity of the earth's orbit may have exercised 

 on the physical condition of our planet. I have 

 ventured, in the Appendix, to discuss some portions 

 of the vast range of subjects treated of by Dr. Croll,* 

 and to state the reasons which force me to dissent 

 from some of his conclusions ; but I shall here merely 

 say that the impressions derived from my own short 

 experience have been confirmed by subsequent diligent 

 inquiry, and especially by the writings of Dr. Julius 

 Hann, most of which have been published since my 

 return to England. 



The belief that the mean temperature of the southern 

 is considerably lower than that of the northern hemi- 

 sphere was, until recently, prevalent among physical 

 geographers, and has been assumed as an undoubted 

 fact by Dr. Croll. He accounts for it by the pre- 

 dominance of warm ocean currents that pass from the 

 southern to the northern hemisphere within the tropics, 

 and which, as he maintains, ultimately carry a great 

 portion of the heat of the equatorial regions to the 

 north Temperate and Frigid Zones. I think that 

 this belief, as well as many others regarding physical 

 geography, originated in the fact that physical science 

 in its more exact form, had its birth in Western Europe, 

 a region which, especially as to climate, is altogether 

 exceptional in its character. The further our know- 

 ledge, yet too limited, has extended in the southern 

 * See Appendix B. 



