242 CAUSES WHICH LIMIT 



is evident tliat it is not heat that is wanting in Brittany to a plant 

 which grows in Scotland. 



On- the continent the Alyssum calycinum spreads to the northwest 

 as far as Holstein and the Baltic, on the northeast as far as Moscow, 

 hut not to Kasan. The limit in the quarters where its extension may 

 he thought to he determined solely hy temperature stretches, there- 

 fore, from Arbroath, in Scotland, under the 56| degree of latitude, 

 passes along the 54th degree in Holstein, and thence oscillates in 

 Russia between the 56th and 55th degree. I shall not stop to show in 

 detail how much this line varies from any isothermal line, isotheral, 

 or other founded on equality of temperature. Comparing only the two 

 extremities, Arbroath, in Scotland, with an annual average of 8°, 

 Moscow of 4°. 5, Arbroath with a summer average of 14°, Moscow 

 of 1T°.8, it will be seen that the mean of each month varies strikingly. 

 I consult laj table of the correspondence of climates, and I find that 

 at Kinfauns, in Scotland, very near Arbroath, the temperature of 7° 

 or upwards continues from the 18th of April to the 31st of October, and 

 that during this time the product of the number of days by the mean 

 temperature amounts to 2281°. At Konigsberg the temperature of 

 7° and upwards is of shorter duration, but the summer being hotter, 

 the product amounts to 2308°. As the limit of the species is about 

 twenty leagues to the north of Konigsberg, that figure must be 

 reduced, and becomes identical with that of Scotland. At Moscow 

 the mean of "7° commences the 22d of April, and terminates the 5th of 

 October ; the product, in consequence of the heat of summer, rises to 

 24*73°. This is more than seems necessary to the Alyssum, and I am 

 induced to believe that it may live thirty or forty leagues to the north 

 of Moscow, but there is no local flora to furnish the assurance of it. 

 At Kasan, the figures fall to 2196°, so that it is not surprising that 

 the species there disappears. Thus the hypothesis of T° of initial 

 temperature and of a product of 2280° to 2300° accords completely 

 with the facts. 



I shall cite another example derived from a ligneous species. 



The Euonymus europa3us has for a limit the north of Ireland, Edin- 

 burgh, (56^- deg. latitude,) the north of Denmark, the south of Swe- 

 den, (57 — 58 lat.,) the isle of Aland, at the entrance of the Gulf of 

 Bothnia, (60 lat.,) Moscow, Pensa, (52 lat.) Tliis limit varies 8° 

 of latitude. In its course the mean annual temperatures vary 4°, 

 the mean winter 12°. 7, and the mean summer 3°. 4. Those from 

 March to November, which are more conformable, still vary to the 

 extent of 1°.5, and the species, moreover, disappears at several points 

 where the mean of this period of the year is overpassed. The average 

 from April to October and of each other period of the year does not 

 coincide more closely. It is necessary, then, to renounce this mode of 

 explanation. But an hypothesis is at hand which accords with all 

 the facts. The Euonymus europa3US requires a product of 2480° be- 

 tween the two epochs of the year when the curve of mean tempera- 

 ture ascends above 6°. In effect this product at Edinburgh is 2482°. 

 In Sweden the city of Stockholm is situated beyond the limit, the 

 product there being but 2268°. St. Petersburgh, with a product of 



