VEGETABLE SPECIES. 241 



imum necessary to eacli species for each of its functions, but botanical 



feogra'>hy will furnish us the means of doing so, if, as I propose to 

 emonstrate, the limits of species depend at the same time on the 

 quantity of heat and the minimum required for each species. And 

 here I enter on a field which has not hitherto been explored. 



An example will enable us to comprehend how the two principles 

 of which I am speaking combine in European climates, and brino- 

 about a similitude or dissimilitude to which the means ordinarily 

 employed furnish no key. 



London and Odessa are certainly not under the same lines of tem- 

 perature. The mean of summer heat is at London 16°. 7, at Odessa 

 20°, while in winter the difference of the mean is much greater. In 

 their monthly mean these two climates have no analogy. Notwith- 

 standing, if we consider the time at which the temperature of 4°. 5 

 oommences and terminates in each of these cities, and the product 

 which represents the heat between these two limits, we find the same 

 figure. At London the mean of 4°. 5 commences the 17th of February, 

 and terminates the 15th of December. Between these two periods the 

 figure expressing, according to the process of M. Boussingault, the 

 heat received, is 3431°. At Odessa the temperature of 4°. 5 com- 

 mences later, from the 2d to the 3d of April, and terminates sooner, 

 from the 17th to the 18th of November, but as it is warmer during 

 summer, the amount of the temperature between those limits is almost 

 equal to that of London, for it is 3423°. Hence a plant which would 

 require 4°. 5 to commence vegetating with a certain activity, which 

 should arrive at the same condition, and would require in all an 

 amount of heat of 3430°, might advance in a northwest direction to 

 London, and in a northeast to Odessa. If a plant should require 

 more or less than 4°. 5 as a minimum, or more or less than 3430° in 

 the whole, the climates would no longer correspond, and the limit of 

 species would be otherwise established. 



This shows us how two European climates, which differ when con- 

 sidered, as regards their respective mean monthly temperature, may 

 yet be identical under certain combinations of the two causes which 

 exert an influence on the life of species. For the purpose of discover- 

 ing these correspondences of climates I have calculated for a certain 

 number of the cities of Europe on what days the temperature of 1°, 2°, 

 3°, &c., up to 8°, commences and ends. I have placed over against 

 this list the product indicating the heat received over and above each 

 of those degrees in all the localities. The application of these figures 

 t-o the facts of vegetation is highly satisfactory, notwithstanding cer- 

 tain sources of error impossible wholly to avoid. I shall here cite but 

 two examples. 



The Alyssum calycinum is a cruciferous annual, which grows here 

 and there on the eastern coast of Great Britain and as high as Edin- 

 burgh, and even a little beyond, as far as Arbroath. It is found neither 

 on the western coast of England nor in Ireland, nor yet in Brittany or 

 Calvados ; but this must be attributed to the constant humidity of 

 those regions, for the Alyssum calycinum prefers a dry region, and it 



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