18431882] GLACIAL PERIOD 463 



periods helps us much. The supposition of the whole world Letter 352 

 being cooled contemporaneously (but perhaps not quite 

 equally, South America being less cooled than the Old 

 World) seems to me the simplest hypothesis, and does not 

 add to the great difficulty of all the tropical productions not 

 having been exterminated. I still think that a few species 

 of each still existing tropical genus must have survived in 

 the hottest or most favourable spots, either dry or damp. 

 The tropical productions, though much distressed by the 

 fall of temperature, would still be under the same conditions 

 of the length of the day, etc., and would be still exposed 

 to nearly the same enemies, as insects and other animals ; 

 whereas the invading temperate productions, though finding 

 a favouring temperature, would have some of their conditions 

 of life new, and would be exposed to many new enemies. 

 But I fully admit the difficulty to be very great. I cannot 

 see the full force of your difficulty of no known cause of 

 a mundane change of temperature. We know no cause of 

 continental elevations and depressions, yet we admit them 

 Can you believe, looking to Europe alone, that the intense 

 cold, which must have prevailed when such gigantic glaciers 

 extended on the plains of N. Italy, was due merely to changed 

 positions of land within so recent a period ? I cannot. It 

 would be far too long a story, but it could, I think, be clearly 

 shown that all our continents existed approximately in their 

 present positions long before the Glacial period ; which seems 

 opposed to such gigantic geographical changes necessary 

 to cause such a vast fall of temperature. The Glacial period 

 endured in Europe and North America whilst the level of 

 the land oscillated in height fully 3,000 feet, and this does 

 not look as if changed level was the cause of the Glacial 

 period. But I have written an unreasonably long discussion. 

 Do not answer me at length, but send me a few words 

 some time on the subject. 



I have had this copied, that it might not bore you too 

 much to read it. 



A few words more. When equatorial productions were 

 dreadfully distressed by fall of temperature, and probably by 

 changed humidity, and changed proportional numbers of other 

 plants and enemies (though they might favour some of the 

 species), I must admit that they all would be exterminated 



