VEGETABLE SPECIES. 243 



1894°, is likewise beyond it. The isle of Aland, where, as we are 

 told, the species grows, may well have a more elevated figure than the 

 neighboring cities of Stockholm and St. Petersbiirgh, in virtue of the 

 influence of the sea, but we possess no observations as to this point. 

 At Moscow the product exceeds somewhat the supposed condition, 

 being 2524° ; but probably, also, the species advances a little toward 

 the north of that city at a point where express information again fails 

 lis. Finally, it is not found at Kasan, and here also the product is 

 only 2250°. The values then found along the limit, in its neigh- 

 borhood and beyond it, accord as nearly as. could be desired in such a 

 matter with our double hypothesis of 6° and 24 80°. 



The chartreux pink, (Dianthus carthusianorum,) a perennial species, 

 is arrested at the west by humidity, but from Koningsberg to Kasan, 

 where its limit depends upon temperature, it is requisite that the 

 plant should receive 2450 degrees between the day when the mean of 

 5 degrees commences and that when it terminates. 



When hypotheses of this character are thus verified successively in 

 many separate cases, and when they repose moreover on incontestable 

 principles of physiology, it may be assumed that they correspond with 

 a law of nature. 



That law may, in the present case, be enunciated in the following 

 terms : Every species Itaving Us polar limit in central or northern Europe 

 advances as far as it finds a certain fixed amount of heat, calcnlated from 

 the day when a certain mean temperature commences to the day when that 

 mean tei'minates. 



The apparent exceptions to this rule may be explained by two cir- 

 cumstances, which restrict its application. 



1. Many species, even in our temperate or northern climates, are 

 influenced as to a portion of their limit by humidity and dryness more 

 than by the conditions of temperature. Those which shun the dryness 

 have a limit inclining from the northwest to the southeast — the east- 

 ern part of the continent being the dryest. The species which shun 

 humidity have a limit inclining from the northeast to the southwest, 

 because the more humid regions are, of course, those lying toward th'e 

 ocean. These causes often determine the west and east limits of 

 species. Quite frequently the same species will be found limited to the 

 east and west by circumstances of this kind, and to the north by the 

 operation of the law above stated. In calculating therefore the figures 

 deduced from temperature, we must ascertain the point at which 

 the limit ceases to be regulated by one of the accessory causes and 

 tails v/ithin the control of the law of temperature. 



2. The perennial species, and, above all, the ligneous ones, are 

 sometimes arrested towards the nortli by the absolute minima of tem- 

 perature. The limit inclines in this case from the northwest to the 

 southeast, because the intense cold prevails most in the interior of the 

 continent. In tracing a limit of species from west to east, if the law 

 as stated ceases to be applicable, the species may be regarded as having 

 encountered the action either of severe cold or of drought; and it is 

 often difficult to discriminate which of these two causes operates as an 



