of Secular Changes of Climate. 89 



ditions of the planet must be wholly different from those of 

 the earth. 



When we reflect that a very slight change in the properties 

 of aqueous vapour, or in the condition of our atmosphere, 

 would effectually prevent the possibility of a glacial epoch 

 occurring on our earth, notwithstanding a high state of eccen- 

 tricity, we need not wonder that the planet Mars is not in a 

 state of glaciation. But the eccentricity of Mars, though 

 high, is still far from its superior limit, and the planet may 

 yet, for any thing which we know to the contrary, pass through 

 a glacial epoch. 



3rd. Another prevailing misapprehension is the supposition 

 that the theory does not recognize the necessity for geogra- 

 phical conditions. In reading ' Island Life ' one might be 

 apt to suppose that one of the chief points of difference be- 

 tween Mr. Wallace and myself is that he regards geographical 

 distribution of sea and land as an important factor in a theory 

 of geological climate, whereas I entirely ignore this condition. 

 Nothing could be further from the truth than such a suppo- 

 sition. I can boldly affirm that the necessity for geographical 

 conditions is as truly a part of my theory as of Mr. Wallace's 

 modification thereof. 



One of the most important agencies, according to my view, 

 is the enormous amount of heat conveyed from equatorial to 

 temperate and polar regions by means of ocean-currents, and 

 the deflection of this heat, during a high state of eccentricity, 

 from the one hemisphere to the other. But all this depends 

 on ocean-currents flowing from equatorial to polar regions; 

 and the existence of these currents in turn depends, to a large 

 extent, on the contour of the continents and the particular 

 distribution of sea and land. Take, as one example, the Gulf- 

 stream, a current which played so important a. part in the 

 phenomena of the- glacial epoch. A very slight change in 

 geographical conditions, such as the opening of communica- 

 tion between the Gulf of Mexico and the Pacific, would have 

 greatly diminished, if not entirety destroyed, that stream. Or, 

 as I showed on a former occasion, a change in the form or 

 contour of the north-east corner of the South- American con- 

 tinent would have deflected the great equatorial current, the 

 feeder of the Gulf-stream, into the Southern Ocean and away 

 from the Carribean Sea. One of the main causes of the ex- 

 treme condition of things in North-western Europe, as well as 

 in eastern parts of America, during the glacial epoch, was a 

 large withdrawal of the w T arm waters of the Gulf-stream; and 

 this was to a great extent due, as I stated in my very first 

 paper on the subject *, to the position of Cape St. Roque, 

 * Phil. Mag. for August 1864. 



