400 
NATURE 
[.Fed. 17, 1870 
the question, how far variations in the excentricity of 
the earth’s orbit, together with the precession of the 
equinoxes, have produced variations of climate in past 
ages. He has endeavoured to show that the date of 
the last glacial epoch and those preceding, may be deter- 
mined by such considerations. With this view he has 
laboriously calculated tables showing the amount of 
excentricity for a period of three million years, at intervals 
of 10,000 years for a large portion of that time, and 50,000 
for the remainder. These tables show that the amount of 
excentricity is alternately great and small at intervals of | 
| organic remains. 
50,000 or 100,000 years, as represented with sufficient 
accuracy in the diagram, which I have constructed by 
means of his figures. Owing to the precession of the equi- 
noxes, combined with the revolution of the apsides, either | 
pole will be presented towards the sun (constituting 
summer in that hemisphere and winter in the opposite 
one) at a different point in the earth’s orbit on each 
succeeding year, the motion being such as to cause a 
complete revolution in 21,000 years. If, therefore, at any 
one period, winter in the northern hemisphere occurs 
when the earth is nearest the sun or in ferthelion (as is 
the case now), in 10,500 years it will occur in aphelion ; 
at the one period the winters will be shorter and warmer, 
at the other longer and colder. 
is great (say two, three, or four times what it is now), 
f 
iV 
When the excentricity | 
million years probably includes a large portion of the tertiary 
period, which therefore should have mainly consisted of 
alternations of warm and cold climates in each hemi- 
sphere, the latter generally forming true glacial epochs. 
This seems the legitimate deduction from Mr. Croll’s 
reasoning and from the tables of excentricity with which 
he has furnished us ; but, as he very justly argues, we 
cannot expect to find geological evidence of all these 
changes of climate. The warm and temperate periods 
will naturally leave the best records, while the cold epochs 
will generally be characterised only by an absence of 
Besides, we must consider 10,500 years 
as a very small fragment of time in geology and we have 
good reason for thinking that several such periods might 
pass away without the occurrence of those exceptional 
conditions which Mr. Darwin and Sir C, Lyell have 
shown to be necessary for the preservation of any geological 
record. As to physical proofs of ice-action, very few 
could survive the repeated denudations, upheavals and 
subsidencies, which the surface must have undergone 
since any of the earlier glacial epochs ; so that it may be 
fairly argued that these repeated changes of climate may 
have occurred and yet have left no distinct record by 
which the geologist could interpret their history. 
Throughout the whole of his argument, Mr. Croll 
considers astronomical causes to be the most important 
9 
Z A Z 
SAMillion Years. 
7 
Mr. Croll shows that, from the known laws of heat 
in reference to air and water, winter in aphelion will 
lead to an accumulation of snow, in the polar regions, 
which the summer will not be able to melt. This will 
¢o on increasing for many thousand years, till winter occurs 
near the ferihelion, when the snow will be melted and 
transferred to the opposite pole. When the excentricity 
was very great a glacial epoch would occur in each hemi- 
sphere for more or less than 10,500 years, the other portion 
of the period of 21,000 years being occupied by an almost 
perpetual spring, with two transition periods from that to the 
glacial epoch. By examining the diagram of excentricity, 
we see that during the last three million years there have 
been more than twelve periods of great excentricity, each 
long enough to admit two or three, and several of them 
eight or ten, complete revolutions of the equinoctial points, 
thus sufficing for the production of not less than fifty or 
sixty glacial epochs in each hemisphere, with intervening 
phases of perpetual spring or summer. 
The diagram also shows us (and this is of very great | 
importance) that the present amount of excentricity is 
exceptionally small. During the last three million years 
there have only been five occasions, always of very short 
duration, when it has been less than it is now, while 
periods of high excentricity have often lasted for two 
hundred thousand years at a time. This period of three 
and effective agents in modifying climate, while Sir 
Charles Lyell maintains that the distribution of land 
and water, with their action on each other by influencing 
marine and aerial currents, are of prepondering import- 
ance. He has certainly shown that these causes have an 
immense influence at the present time. The effects 
which, on Mr. Croll’s theory, ought to be produced by the 
existing phase of precession combined with even the small 
amount of excentricity that now exists, is not only neu- 
tralised, but actually reversed by terrestrial causes. Dove 
has shown that the whole earth is really warmer when it 
is furthest from the sun in June, than when it is nearest in 
December, a fact which is to be explained by the northern 
hemisphere (turned toward the sun in June) having so 
much more land than the southern. So, the northern 
hemisphere being three millions of miles nearer the sun in 
winter than in summer, while the southern hemisphere is 
the reverse, the northern winter ought to be warmer and 
the northern summer cooler than the southern ; but this, 
too, is the opposite of the fact, for the southern summer is 
more than 11° Fahr. cooler than ours, while its winter is 
nearly 5° Fahr. warmer. ‘The immense differences of 
temperature of places in the same latitude, sometimes 
amounting to nearly 30° Fahr., can also be traced, in 
almost every instance, to the distribution of land and 
water and of windsand currents. Sir Charles Lyell further 
