480 X. GENERAL REMARKS. 



200 days at or averaging 30 fahr. make 6000. 



150 „ 40 „ 6000. 



100 „ 60 „ 6000. 



50 „ 120 „ 6000. 



25 „ 240 „ 6000. 



This sort of reduction into absurdity, however, is avoided 

 by a limitation of the temperatures to those useful or 

 necessary for the species, omitting extremes. All the 

 temperatures below those which promote the due develop- 

 ment of the species, and all those which are needlessly or 

 injuriously too high, are to be left out of the reckoning. 

 The necessity of these omissions renders the problem too 

 intricate for practical solution and service. The sum of 

 the useful temperatures will be found scarcely reckonable 

 for any species. And no ordinary thermometrical records 

 are fairly available, on account of the variable rates and 

 degrees required by the different species of the same flora 

 or country, and differently required by the same species 

 at their different stages of gi"owth. It is highly probable 

 also, that different temperatures are requisite in some de- 

 gree of relation to differences of humidity ; and thus fur- 

 ther increasing the difficulties of the subject. It would 

 seem hardly less difficult to ascertain the gross amount 

 of atmospheric motion, which is required by any given 

 species of plant, or is endurable by it without deteriora- 

 tion or destruction. 



The established routine of mean annual temperature 

 has been adhered to in this work, not because tliis is 

 believed to be the truest or closest indication of the cli- 

 mate required by a species, but because it is the only 

 indication practically available at present. The degree 

 of mean temperature, at which a species ceases to occur 

 native in this country, is certainly no measure of that at 



