ANNIVERSARY ADDRESS OF THE PRESIDENT. Ixiii 



of sea and land. And j^et, granting the most favourable circum- 

 stances, and assuming that, instead of the present irregular and 

 unequal 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 undoubt- 

 edly 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. 



Prof. Heer, like many others, is much tempted by the ingenious 

 inquiries of Mr. James CroU on the results of the varying excen- 

 tricity 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 apjiroximation to that figure, and the excen- 

 tricity will be at its minimum, or little above half a million of miles. 

 At the present time the linear value of the excentricity is three 

 millions ; and w^hen 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 hemisphere, 

 and furthest during our summer. But since, in the meanwhile, 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 northern summer will, in about 10,000 years, coin- 

 cide 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 suffer 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 con- 

 tinual spring. It has been calculated that such a concurrence of 

 these elements of position took place 850,000 years ago, giving 36 

 days of winter in excess, a mean temperature in the latitude of 

 London of 126° E. for the hottest, and —7° for the coldest month, 

 and when it appears probable that the Glacial period was in force, 

 — although only 50,000 years earlier, when the excentricity was 

 at a minimum, the climatal conditions must have been entirely 

 reversed*. 



Whilst, however, Prof. Heer leans to the opinion that some effect 

 from these latter causes may have combined with that of geogra- 

 phical 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 assistance, 

 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 



* For a full discussion of the causes of vicissitudes of climates vide Ly ell's 

 * Principles,' 10th edit. chap. xii. & xiii. 



