GEOGRAPHY AND EVOLUTION. 199 



these, with the periodical winds, tend ou the one hand to equalize the 

 temperature of the whole surface of the earth, and on the other to 

 cause surprising variations within a limited area. Ranges of moun- 

 tains, and their position in relation to the periodical or rain-bearing 

 winds, are of primary importance in controlling the movements of 

 the lower strata of the atmosphere, in which, owing to the laws of 

 elastic gases, the great mass of the air and watery vapor are concen- 

 trated. By their presence they may either constitute a barrier across 

 which no rain can pass, or determine the fall of torrents of rain around 

 them. Their absence or their unfavorable position, by removing the 

 causes of condensation, may lead to the neighboring tracts becoming 

 rainless deserts. 



The difficulties that arise, in accounting for the phenomena of cli- 

 mate on the earth as it now is, are naturally increased when the 

 attempt is made to explain what is shown by geological evidence to 

 have happened in past ages. The disposition has not been wanting to 

 get over these last difficulties by invoking supposed changes in the 

 sources of ten-estrial heat, or in the conditions under which heat has 

 been received by the earth, for which there is no justification in fact, 

 in a manner similar to that in which violent departures from the ob- 

 served course of Natui-e have been assumed to account for some of 

 the analogous mechanical difiiculties. 



Among the most perplexing of such climatal problems are those 

 involved in the former extension of glacial action of various sorts 

 over areas which could hardly have been subject to it under existing 

 terrestrial and solar conditions ; and in the discovery, conversely, of 

 indications of far higher temperatures at certain places than seems 

 compatible with their high latitudes ; and in the alternations of such 

 extreme conditions. The true solution of these questions has appar- 

 ently been found in the recognition of the disturbing efiects of the 

 varying eccentricity of the earth's orbit, which, though inappreciable 

 in the comparatively few years to which the affairs of men are limited, 

 become of great importance in the vastly increased period brought 

 into consideration when dealing with the history of the earth. The 

 changes of eccentricity of the orbit are not of a nature to cause ap- 

 preciable differences in the mean temperature either of the earth gen- 

 erally or of the two hemispheres ; but they may, when combined with 

 changes of tlie direction of the earth's axis caused by the precession 

 of the equinoxes and nutation, lead to exaggeration of the extremes 

 of heat and cold, or to their diminution ; and this would appear to 

 supply the means of explaining the observed facts, though doubtless 

 the detailed application of the conception will long continue to give 

 rise to discussions. Mr. Croll, in his book entitled " Climate and 

 Time," has recently brought together with much research all that can 

 now be said on this subject ; and the general correctness of that part 

 of his conclusions which refers to the periodical occurrence of epochs 



