uncertain and so complicated that it is
difficult to estimate their real value. Another is that it proves too
much, namely, a regular succession of cold and warm periods throughout
geological time, of which we have no good evidence, and which is on
many grounds improbable.

(2.) That the earth's axis of rotation has continued unchanged
throughout the whole of the geological ages seems proved by the fact
that the principal lines of crumpling and upheaval from the Laurentian
period downward are arranged in great circles of the earth tangent to
the polar circle; and that the lines of deposit of sediment in the
Palaeozoic age are coincident with the present direction of the arctic
currents.

(3.) Astronomers consider it improbable that the obliquity of the
ecliptic has materially changed, and serious differences of opinion
exist as to the effects which a greater or less obliquity would
produce on climate. It seems certain, however, that a less obliquity
would occasion a more uniform distribution of heat and light
throughout the year; and this, co-operating with other causes leading
to a warm climate, might enable a temperate vegetation to approach the
pole more closely than at present.

(4.) That the energy of the sun's radiation and the internal heat of
the earth have been slowly decreasing seems certain; but it is now
generally admitted that these changes are so gradual that little
effect can have been produced by them, except in the older geological
periods, and that they can have no connection with the great glacial
period of the Post-pliocene.

(5.) It is otherwise with the hypothesis that the sun's heat may, like
that of some variable stars, have increased and diminished. There is,
of course, no direct evidence of this, except the small differences
observed in cycles of eleven and fifty-five years from the greater or
less development of sunspots, and the analogy of observed variable
stars. Still it is a possible cause of variations of climate. It might
also aid in accounting for the extraordinary evidences of desert
conditions and desiccation presented by the salt deposits of different
geological periods in temperate latitudes.

(6.) The theory of the passage of the earth through zones of space of
variable temperature is now generally abandoned, as there seems no
reason to believe that such differences exist.

(7.) The theory of Lyell that changes in the distribution of land and
water may, with the possible c