68 Prof. O. Heer on the Miocene Flora 
been a continual spring, the summer being long and cool, and 
the winter short and warm. Mr. Stones has calculated that 
we must go back 850,000 years to reach the eg at which 
the eccentricity of the orbit of the earth attained its maximum 
value, at the same time that the aphelion coincided exactly 
with the winter solstice in the northern hemisphere. The 
winter would then have lasted thirty-six days longer; and as 
it is at this period that the greatest quantity of ice and snow 
would have been formed, Lyell is inclined to place in it the 
glacial epoch. But 900,000 years ago, on the other hand, 
the orbit of the earth would have most nearly approached the 
circular form, and from this would have resulted: a complete 
change of climatic conditions. } 
All these speculative theories are certainly ingenious, but it 
must be remarked that they have not a solid basis; in fact we 
still only very imperfectly know what is the extent of the action 
which might be exerted upon the power of the rays of the 
sun by the distance which they traversed to arrive at the 
earth. Lyell has pointed out, with reason, that according to 
Dove’s bal elation the earth is hotter in July (that is to say, at 
the moment when it is most distant from the sun) than in 
December (when it most nearly approaches it). The cause of 
this is the unequal distribution of sea and land in the two 
hemispheres, from which it results that the northern hemi- 
sphere has a hotter summer, even when the earth is nearest to 
the sun during the summer of the southern hemisphere. From 
this fact we may conclude that the mode of distribution of the 
sea and land on the surface of the globe exerts a greater in- 
fluence upon the climate of each hemisphere than that which can 
result from the greater or less eccentricity combined with thie 
position of the line of the apsides. On the other hand, however, 
as Lyell has admirably demonstrated, these two causes, by the 
combination of their effects, may have had an extremely im- 
portant influence upon the changes of climate sake the 
observed facts enable us to demonstrate. 
It is also possible that the action of the sun has not always 
been the same; for, by the observation of its spots, we know 
that great modifications take place upon its surface, whence 
the possibility of a change in the intensity of the solar rays. 
To all these considerations this one may be added :—The 
sun is not alone in the vault of heaven; millions of celestial 
bodies likewise shine there and diffuse their light and heat 
into space. Why, then, may we not suppose that the different 
regions of space have not all the same temperature? The 
mathematician Poisson put forward this idea, by calling atten- 
tion to the fact that the number of stars is so-great that they 
form, as it were, a continuous vault. We also know that the 
