278 Ilecr — Miocene Flora of the Polar Regions, 



Great stress was some time since laid on the internal heat of the 

 globe, and it was thought that this might account for the higher 

 temperature of the early ages of the world. But even if this may 

 with some probability be thought to apply to the oldest periods, it 

 cannot do so to the Miocene times, for they come so near to our own 

 age, that we cannot venture to attribute to such a cause so great a 

 difference of temperature. It is, therefore, not possible to explain 

 this great change of climate from the conditions of our globe, at any 

 rate from those which are at present known to us. 



We must, therefore, turn to cosmical conditions, and see whether 

 we can find in them the solution of the enigma. We may take into 

 consideration the changes in the position of the earth relative to the 

 sun — in the intensity of the sun's rays, and in the temperature of the 

 universe. 



With respect to the first, great stress has lately been laid on the 

 periodical changes in the eccentricity of the earth's orbit. It is well 

 known that this is not a circle, but that it forms an ellipse, in con- 

 sequence of the influence of the larger planets. The form of this 

 ellipse changes within certain limits in the course of thousands of 

 years. At the present moment the earth's orbit is gradually ap- 

 proaching the form of a circle, and in 23,900 years the eccentricity 

 will have reached its minimum, and become most like a circle, but 

 from that time it will gradually become more eccentric. The mean 

 distance of the earth from the sun is 91,400,000 English miles ; the 

 greatest eccentricity of the orbit is about 1-1 3th of this distance, 

 while the smallest is l-360th. At the time of its greatest eccentricity 

 the earth would be about W\ millions of miles further from the sun 

 than when its orbit most nearly approaches a circle. At the present 

 time the difference amounts to 3 millions of miles. We must further 

 bear in mind, that at the present time the earth in the winter of 

 the northern hemisphere is nearest the sun (in perihelion), and in 

 Bummer it is furthest from it (in aphelion). But even this condition is 

 subject to a periodical change, which runs its course in 21,000 years. 

 In about 10,000 years hence the summer of the northern hemis- 

 phere will coincide with the time when the earth is nearest the sun, 

 and the winter with the time when it is furthest from it ; while, of 

 course, these conditions will be reversed in the other hemisphere. It 

 has therefore, been assumed that at those periods when the earth has 

 reached its maximum eccentricity, and when it also is nearest to the 

 sun in winter (or in perihelion), this hemisphere has had a shorter 

 and warmer winter, but on the other hand a longer and cooler 

 summer; while the southern hemisphere must at this period have 

 had exactly the reverse, that is a longer and colder winter, and a 

 warmer and shorter summer, because the greatest distance from the 

 sun must coincide with the winter of this hemisphere. Mr. CroU 

 supposes that during this longer and colder winter so much ice must 

 have formed, that the short summer, though certainly warm, would 

 not have been able to melt it, and that the glacial period was a 

 consequence of these conditions. During this period a perpetual 

 spring would have prevailed in the other hemisphere, for the long 



