130 EEV. G. HENSLOW — OKIGIN AND DISTRIBTJTION 



added, for 'Nam, on the cocast of Labrador, it is 50°, and for Cape 

 Churchill, on the west coast of Hudson's Bay, the difference is 

 even 80°. All the above places are very nearly on the same parallel 

 of latitude. Again, if we take winter and summer temperatures, 

 we find that for July the mean at London is over 62° ; at Berlin, 

 66° ; at St. Petersburgh, 64° ; and at Astrakhan, 77°. While for 

 January at London it is 37° ; at Berlin, 28° ; at St. Petersburgh, 

 1 6° ; and at Astrakhan it is 1 3°. Similarly in Cumberland (North 

 America), in the latitude of Edinburgh, the winter temperature is 

 — 13°, the summer temperature being +62°, 



If we consider the temperature of places in the west of Europe, 

 we soon see how important is the influence of warm aerial currents 

 in regulating and ameliorating them; thus, at Hammerfest (lat. 71°), 

 in Norway, the mean winter temperature is 22°, while in the 

 same latitude in Greenland it is 5 degrees below zero. Again, the 

 temperature at 



Caithness 58 deg. N.L. is 36 deg. in January 



Labrador ,, ,, ,, — 4 ,, 



Lisbon 39 ,, ,,47 ,, 



Chesapeake Bay ,, ,, ,, — 36 ,, 



And the temperature at 



Edinburgh 56 deg. N.L. is 37 deg. in January 



Bergen (jN'orway) 60 ,, ,,32 ,, 



Jakutsk 62 ,, ,, — 36 ,, 



In the latter group of places we see the great contrast between the 

 temperature of an inland site, such as Jakutsk, and that of the 

 maritime coast of the west of Europe, which is swept by warm 

 currents. 



Now, the most obvious effect that such differences of temperature 

 have on plants is that a continental climate is favourable to annuals 

 and a maritime to perennials ; for in places where a summer tem- 

 perature rises high, plants, whose whole life-history is comprised 

 in a few months or even weeks, may easily, therefore, survive ; 

 while the intensely cold winters of the same place would annihilate 

 many perennials that would flourish in a less rigorous climate. 

 Hence evergreen shrubs of South Europe, such as the laurustinus 

 and bay laurel, will survive our winters, which are rarely 

 excessive, yet the climate in summer and autumn is quite insuffi- 

 cient in its degree of heat to ripen efficiently the grape or Indian 

 corn ; for the summers are as equally tempered as the winters. 



The British flora, as might, therefore, be expected, contains a 

 large amount of perennials, especially, perhaps, herbaceous ones. 

 Many annuals, being weeds of cultivation only, would be probably 

 more or less exterminated if our arable land should cease to be 

 cultivated. 



In reviewing our flora as a whole, in some respects it may be 

 regarded as insular in character, thougli in others it is continental ; 

 that is to say, there is no plant which is peculiar to it, and with 

 rare exceptions every member of it belongs to the neighbouring 

 Continent of Europe. As, however, we are at present insular, it 



