126 



NATURE 



[Dec. 12, 1878 



ascertained fact that they require less heat in latitudes 

 above 60°, owing to this rapid lengthening of the days. 

 The chemical action of the sun's rays seems in some 

 way to compensate for feeble warmth, and vegetation 

 receives more impulse from the presence of the sun than 

 from temperature in the shade. As examples of this, 

 De CandoUe ' mentions that Fagus sylvatica exists 

 in the north with a less temperature than it can sup- 

 port elsewhere ; and that the limits of growth of barley 

 prove the point conclusively. 



It appears certain, according to De Candolle, that in 

 very few cases has even intense cold, during natural periods 

 of rest, any injurious influence upon plants, and that their 

 northern limits are not determined by excess of cold but by 

 want of heat. The destructive agents are late spring frosts, 

 or premature heat followed by chills ; and so fatal are 

 their effects that one week in May has killed entire stocks 

 of sub-tropical plants which had stood considerable frost in 

 winter. There is no doubt that many plants would grow 

 in much colder latitudes if the temperatures of each 

 month were cyclically regular. Fraximts excelsw}', L,,» 

 supports great cold, especially when accompanied by 

 fogs, and penetrates as far north as 64°. Ilex aquifolucm, 

 L., reaches latitude 62° in Norway, and, like Abies, is 

 limited in range, not by excess of cold, but want of 

 heat. Evofiyinus europcsus, L., is found just within 60°, 

 and must occasionally suffer intense cold. But perhaps 

 the well-kno\vn Chmncerops humilis, L., affords the most 

 striking familiarly-known instance of capricious distribu- 

 tion. It is indigenous to Nice in latitude 40^, yet it is 

 not found anywhere in Italy, with a trifling exception, 

 until Calabria is reached. Under cultivation it bears a 

 very considerable amount of winter frost, the limit of 

 which I have not ascertained, nor the minimum it en- 

 counters at Nice. I merely mention these instances as 

 indicating possible sources of error, for were Cham serops 

 extinct and found fossil at Nice, we should infer from it, 

 with every appearance of probability, that the tempe- 

 rature of Nice had been the same as that of Sicily or 

 Granada, the more normal homes of the palm. 



One of the most remarkable facts connected with Alpine 

 or Arctic plants is the length of time they can endure the 

 absence of light while they are covered with snow, and 

 when thus protected they would be unaffected by even 

 Arctic cold. Evergreens, as we see by the Alpine rhodo- 

 dendron, are equally unaffected, and I have in Switzer- 

 land seen laurels, bays, and acubas shrouded in snow 

 for many weeks without injury. I will mention but one 

 other instance of the extent to which trees will sometimes 

 bear cold, quoted by Herschel.^ "In the valley of the 

 black Irkut, in Siberia, Atkinson found a ravine filled 

 with ice, and with Jarge poplars growing in it, with their 

 trunks imbedded 25' in snow and ice, while the branches 

 were in full leaf. Around each stem was a hollow of 6" 

 thawed and full of water." Besides mere heat and cold 

 there are many influences known and unknown which 

 limit the range of plants. The distribution of the vine is 

 a case in point, for it is well kown that in historic times it 

 was extensively cultivated in England, Normandy, and 

 parts of Prussia, in which it will no longer ripen its fruit. 



While the winter temperatures in these Arctic regions, 

 if accompanied by snow and fogs, may have been of 

 extreme severity, the summer temperature need not have 

 been high, for the present Arctic and Alpine plants, 

 including roses, species of Betula, Salix, Empetrum, 

 Vaccinium, and conifers need but little heat. 



Having attempted to show that the amount of heat 

 really required was not so large as has been imagined, I 

 will endeavour to prove that it, even upon Heer's as- 

 sumption, might easily have been furnished by physical 

 causes which we know did, in all probability, exist in 



De Candolle, " Gdog. Botanique," vol. i., 1853. 

 •• Examples used by De Candolle. 

 3 " Physical Geography," p. 312. 



eocene time, and were quite independent of astronomi- 

 cal causes and change in the position of the earth's 

 axis, of which there seems to me no proof whatever, 

 geological or otherwise. At least, to qualify this assertion, 

 if such have existed in the past, there is no need to 

 invoke them in this particular case. Central heat may, 

 of course, be dismissed as having had too little influence 

 in eocene time to be appreciable. 



We may roughly estimate, on Heer's basis, that the 

 average temperature between the latitudes and longitudes 

 of England and Iceland was not more than from 15° F. 

 to 20° F. warmer in eocene (or miocene of Heer) times than 

 it is at present, and we may assume also on the evidence 

 we possess that the present climate would permit any of 

 the eocene floras, supposing they still existed, to grow in 

 latitudes not more removed from those in which they are 

 found than 15° to 20° farther south. For example, the 

 English eocene flora could now exist in Madeira, the Ice- 

 land eocene flora in the Isle of Wight, that of Spitzbergen 

 in Sweden, and that of Grinnell-land in NorthernNorway. 

 We have therefore to seek for some cause adequate to 

 produce a difference in the temperature of Greenland, for 

 instance, equal in degree to that of 20° F. or 20° latitude 

 as a maximum. Following upon a map the isotherms of 

 the 70th parallel, we see that Prince Albert Land has a 

 temperature of but 5° F., whilst Lapland, in the same 

 latitude, has one of 32° F. There is evidently here a 

 cause at work capable of influencing the temperature to 

 the extent of 27° F. ; therefore a more powerful cause 

 than is required. The same map shows us obviously 

 that this agent is the sea. Wherever the Arctic waters 

 find egress or penetrate the land, the isothermal lines 

 around the Pole are deflected south. In like manner 

 the line denoting the limits of trees is in many places 

 pushed back more than 10° S. by the ice-laden water 

 flowing from the Arctic Ocean. From Lapland ta 

 Siberia it is, except for a short distance, within the 

 Arctic circle, principally within the 70th parallel. Near- 

 ing Behring's Straits it is sharply deflected south 

 by the Polar Sea, but away from its influence, it as 

 suddenly rises and again (North America) far overlaps 

 the Arctic circle, until it once more comes under the 

 influence of the cold seas and channels penetrating south 

 into Hudson's Bay, which drive it to below the 60th 

 parallel. Avoiding Greenland, it includes part of Ice- 

 land and the whole of the North Cape, owing to the 

 influence of the Gulf Stream. The influence of this 

 warmer water, cold as it is here, is no less remarkable, 

 for, by merely shutting oft" the Arctic currents from close 

 proximity to the shore, it enables trees to grow on the 

 coast, and at a point on the Arctic circle between Iceland, 

 and the Norwegian coast, raises the temperature, accord- 

 ing to Herschel, full 20° above that which is normal to 

 the latitude.' 



We thus see that the limit of trees enters the Arctic 

 circle wherever the land has a great extension south or 

 where the Gulf Stream raises the temperature, but that it 

 especially shuns wherever the Arctic waters penetrate 

 the land, even in the smallest gulfs or bays. The lands 

 between Hudson's Bay and Davis' Straits, cut up by 

 water, and the islands in the Arctic Ocean surrounded 

 by water, are intensely cold and destitute of trees — 

 almost of vegetation. The cause of Greenland's being 

 shrouded in ice is its unknown and exceptional extension, 

 towards the Pole and the increased height of land in its 

 northern portion. These appear to be necessar)' condi- 

 tions of such complete glaciation as we there see, as 

 shown by the absence of an ice-cap in Grinnell and other 

 equally northern lands. The present condition of Green- 

 land is wholly abnormal, and, presenting such unusual 

 conditions, has heightened the astonishment felt when, 

 the former mildness of its climate became known. 



If we were able to shut off from the Atlantic the enor- 



* " Physical Geography," p. 232. 



