298 On the Influence of the Gulf Stream. 



distribution of sea and land, we had the continents united near the 

 equator, and only scattered islets left amid great oceans in the 

 higher latitudes, the mean annual temperature would undoubtedly 

 be raised in no small degree, but not sufficiently to admit of the 

 growth of a rich vegetation between the parallels of 70 and 80 

 degrees. The very fact, however, of the wide distribution of this 

 luxuriant Miocene flora shows that a large area of land was then 

 amassed in the temperate and polar zones, and consequently that 

 such explanation is inadequate to account for the facts. 



*' Professor Heer, like many others, is much tempted by the 

 ingenious inquiries of Mr. James Croll, on the results of the vary- 

 ing eccentricity of the earth's elliptical orbit. The present tendency 

 of its course is towards the form of a circle, and in 23,912 years it 

 will have made its nearest approximation to that figure, and the ex- 

 centricity will be at its minimum, or little above half-a-million of 

 miles. At the present time the linear value of the eccentricity is 

 three millions, and when the orbit attains to the opposite extreme 

 of form, it is above fourteen millions of miles. At present, also, the 

 earth is nearest to the sun during the winter of our Northern hemi- 

 sphere, and furthest during our summer. But since, in the mean- 

 while, the relative position of the line of the apsides and that of the 

 solstices is affected by a movement of revolution occupying 21,000 

 years for its completion, our northera summer will, in about 10,000 

 years, coincide with the perihelion, and the winter with the 

 aphelion. Now when this latter coincidence takes place at the 

 time of maximum excentricity of the orbit, the hemisphere so 

 affected must suifer an unusually high degree of cold ; the moisture 

 in winter would be precipitated as snow, and vast masses would 

 be accumulated which the summer's heat would be unable to melt. 



'' The other hemisphere would in the meanwhile enjoy a temperate 

 climate, like a continual spring. It has been calculated that such a con- 

 currence of these elements of position took place 850,000 years ago, 

 giving thirty-six days of winter in excess, a mean temperature in 

 the latitude of London of 126° F. for the hottest, and 7° for the 

 coldest month, and when it appears probable that the Grlacial period 

 was in force, although only 50,000 years earlier, when the excen- 

 tricity was at a minimum, the climatal conditions must have been 

 entirely reversed. 



" Whilst, however, Professor Heer leans to the opinion that some 

 effect from these latter causes may have combined with that of geo- 

 graphical distribution of land and sea to produce changes of climate, 

 and that the latter is probably the more energetic, as it is also 

 the most securely deduced source of action, he looks further for as- 

 sistance, and suggests the passage of our solar system through 

 regions of varying temperature. This hypothesis was examined in 

 detail by Hopkins, in his admirable paper on the causes which may 

 have produced changes in the earth's superficial temperature, more 

 particularly with reference to its being assigned as an explanation 



^ 1 For a full discussion of the causes of vicissitudes of climates, vide Lyell's " Prin- 

 ciples," lOtli edit. chap, xii, and xiii. 



