urred
whenever the excentricity was unusually large. Dr. Croll has published
tables showing the varying amounts of excentricity for three million years
back; and from these it appears that there have been many periods of high
excentricity, which has often been far greater than at the time of the last
glacial epoch.[10] The accompanying diagram has been drawn from these
tables, and it will be seen that the highest excentricity occurred 850,000
years ago, at which time the difference between the sun's distance at
_aphelion_ and _perihelion_ was thirteen and a half millions of miles,
whereas during the last glacial period the maximum difference was ten and a
half million miles.

[Illustration: DIAGRAM SHOWING THE CHANGES OF EXCENTRICITY DURING THE LAST
THREE MILLION YEARS.]

Now, judging by the amount of organic and physical change that occurred
during and since the glacial epoch, and that which has occurred since the
Miocene period, it is considered probable that this maximum of excentricity
coincided with some part of the latter period; and Dr. Croll maintains that
a glacial epoch must then have {60} occurred surpassing in severity that of
which we have such convincing proofs, and consisting like it of
alternations of cold and warm phases every 10,500 years. The diagram also
shows us another long-continued period of high excentricity from 1,750,000
to 1,950,000 years ago, and yet another almost equal to the maximum
2,500,000 years back. These may perhaps have occurred during the Eocene and
Cretaceous epochs respectively, or all may have been included within the
limits of the Tertiary period. As two of these high excentricities greatly
exceed that which caused our glacial epoch, while the third is almost equal
to it and of longer duration, they seem to afford us the means of testing
rival theories of the causes of glaciation. If, as Dr. Croll argues, high
excentricity is the great and dominating agency in bringing on glacial
epochs, geographical changes being subordinate, then there must have been
glacial epochs of great severity at all these three periods; while if he is
also correct in supposing that the alternate phases of precession would
inevitably produce glaciation in one hemisphere, and a proportionately mild
and equable climate in the opposite hemisphere, then we should have to look
for evidence of exceptionally warm and exceptionally cold periods,
occurring {61} alternately and with several repetitions, within a