242 DISCUSSION OF THE DESICCATION QUESTION. 



A lew words must be said, before proceeding farther, as to the way in 

 which the nature of tlie evidence touching the question before us is affected 

 by the demonstrated fact that tlie land surface of the earth has been increas- 

 ing during the geological periods. 



It needs but little study of the subject to make it apparent that geological 

 proof of climatic changes must be furnished chiefly by the land. The well- 

 known physical conditions of water are the cause that the ocean, taken as a 

 whole, is cold ; only the surface is heated by the sun in warm climates ; the 

 great body of the water, even within the tropics, has a temperature at most 

 only a few degrees above the freezing point. Hence, we have but to descend 

 to a moderate depth, even in the warmest climate, to find a fauna indicative 

 of low temperature. It may be said that analogous conditions prevail on the 

 land, where we l)ave only to rise sufficiently in altitude to find the vegeta- 

 tion entirely changed, so that even between the tropics we have an Al2)ine 

 ilora along the higher parts of the mountain ranges. But it must be remem- 

 bered that the high portions of the land occupy an exceedingly small area 

 as compared with the whole surface of the earth, while almost the whole 

 ocean is very deep. Moreover, the lower portions of the land and of the 

 slightly submerged land masses are the regions where takes place the accu- 

 mulation of detritus containing the imbedded fossil remains by means of 

 which we are enabled to arrive at a conclusion as to the temperature of the 

 period when the strata in question were formed ; for the detrital material 

 itself, if destitute of traces of organic life, would be of no assistance to us in 

 determining that point. 



In the case of organic remains enclosed in any assemblage of strata, if 

 they belong to the vegetable kingdom or to land animals, we know that they 

 lived on the surfiice, and that they are therefore indicative of the tempera- 

 tvire of that surface at the time of their growth; but in the case of marine 

 forms we always have the doubt in regard to the depth at which the deposit 

 took place entering in to complicate the inquiry. Moreover, since the earth's 

 surface is everywhere exposed to our examination, we can ascertain the 

 mean temperature of every portion by direct observation, and at the same 

 time find out with ease what forms of vegetable and animal life flourish 

 under the climatic conditions there prevailing. 



From the considerations stated above, it is evident that the farther back 

 we go in geological time the greater the difficulties with which Ave are likely 

 to meet in decidin;.:; on the nature and extent of the climatic changes which 



