VEGETABLE SPECIES. 245 



and different years ; but in this regard we must expect to encounter in 

 its application difficulties arising as well from the nature of tlie facts 

 to be observed as from the variableness of the years.* 



" For the epochs of vegetation, as for heights souiewhat considcrabhj, and for polar re- 

 gions, a circumstance touching the manner of c^^lculating temperatures will render tl;e 

 appliciition of the law rather difficult. In thermometrical averages the ligures below 0°, 

 are made to enter as negatives ; now, it would be necessary, in order to appreciate the 

 action upon plants, to regard them as nought, without retrenching anything from the 

 values above 0°, 1°, 2^, &c. A species which would manifest the effect of a heat of 1°, 

 might vegetate in a season wlien the mean ehould be greatly below 0°, according to the 

 ordinary mode of calculation. It wourd suffice that the thermometer lias been above 0^ 

 for a certain number of hours. This consideration has prevented me from taking for a 

 subject of study species which have their polar limit in Iceland, Lapland, and under other 

 very high latitudes. Tlie meteorological tables give the monthly averages calculated by sub- 

 traction of the negative values ; and even when tlie observations are given in detail, it is 

 difficult, and often impossible, to know during how many hours in a month the thermo- 

 meter has been above each of the degrees. I solicit the attention of calculators to this 

 point. 



Finally, it will be incumbent on zoologists to examine whether the law we have given 

 does not regulate the limits of certain descriptions of animals ; of those especially which are 

 hatched from eggs, or which recover from a hybernating torpor at certuin temperatures, 

 and which thus require, I suppose, for the total of their active life a certain amount of heat. 

 Zoology and botany having always made a parallel progress, it is seldom th;it a law dis- 

 covered in one of these sciences does not immefliately receive its application in the other. 



As for the relations which connect botanical geography with geology, they become every 

 day more numerous. Perhaps geologists will take pleasure in seeing that the mode of 

 action of temperature upon actual species may be stated with precision. Let them permit 

 me, in closing, a reflection concerning the islands which lie in the neighlwrhood of the 

 European continent. It has occurred incidentally in the course of my researches. If it 

 presents no new results, whicli I do not know, it will have at least the advantage of resting 

 on facts foreign to geology itself. 



As regards the British isles, the actual limits of the species which I have examined al- 

 v/a}'s admit of explanation from meteorological causes, without the material obstacle of the 

 sea appearing in the least to influence them. The limits are not the shore of the ocean ; but 

 if a species is wanting to tlie British isles, it is wanting also to the opposite and neighbor- 

 ing coast, especially to Brittany, the climate of which is nearly similar ; if it exists upon 

 the shores of the continent, it exists also in England. We may conclude from this, either 

 that the seeds have been transported without difficulty across the Channel, which is not 

 probable for most of them, or rather that the arm of the sea has been formed within the 

 existence of the actual species. We know that this opinion has been recently maintained 

 by Mr. Forbes, who explains after an analogous manner certain relations between the 

 British islands and remoter countries, sucli as Spain, the Azores, Lapland, &c. With re- 

 gard to the islands of the Mediterranean sea the facts are different. I might cite several 

 cases in which it is impossible to explain the presence or absence of a species on meteorolo- 

 gical reasons. Thus, according to the observations, rather imperfect it is true, of M. de la 

 Marmora, the south of Sardinia presents the same conditions as regards rain and monthly 

 temperature with certain parts of Sicily ; yet many of the species of Sicily are wanting to 

 Sardinia and vice versa. Tlie chamiurops humilis grows in Sardinia and at Yillefranche near 

 Nice ; it fails in Corsica, which is between them. In general, notwithstanding certain evi- 

 dent analogies of vegetation, the islands and peninsulas of the Mediterranean offer numer- 

 ous anomalies in the limits of species. It would seem that this region had been disturbed 

 by many successive geological revolutions since the existence of the vegetables of our epoch, 

 and that the accidental transportation of seeds had been, to this day, insufficient to bring 

 about a conformity between the limits and the climates. 



