shore-deposits";
and even the chalk, which is so often said to be the exact equivalent of
the "globigerina ooze" now forming in mid-Atlantic, is shown to be a
comparatively shallow-water deposit formed in inland seas, or in the
immediate vicinity of land. The general stability of continents has,
however, been accompanied by constant changes of form, and insular
conditions have prevailed over every part in succession; and the effect of
such changes on the distribution of organisms is pointed out.

We then approach the consideration of another set of changes--those of
climate--which have probably been agents of the first importance in
modifying the specific forms as well as the distribution of animals. Here
again we find ourselves in the midst of fierce controversies. The
occurrence of a recent glacial epoch of great severity in the northern
hemisphere is now universally admitted, but the causes which brought it on
are matter of dispute. But unless we can arrive at these causes, as well as
at those which produced the equally well demonstrated mild climate in the
Arctic regions, we shall be quite unable to determine the nature and amount
of the changes of climate which have occurred throughout past ages, and
shall thus be left without a most important clue to the explanation of many
of the anomalies in the distribution of animals and plants.

I have therefore devoted three chapters to a full investigation of this
question. I have first given such a sketch of the most salient facts as to
render the phenomena of the glacial epoch clear and intelligible. I then
review the various suggested explanations, and, taking up the two which
alone seem tenable, I endeavour to determine the true principles of each.
While adopting generally Mr. Croll's views as to the causes of the "glacial
epoch," I have introduced certain limitations and modifications. I have
pointed out, I believe, more clearly than has hitherto been done, the very
different effects on climate of water in the liquid and in the solid state;
and I have {535} shown, by a variety of evidence, that without high land
there can be no permanent snow and ice. From these facts and principles the
very important conclusion is reached, that the alternate phases of
precession--causing the winter of each hemisphere to be in _aphelion_ and
_perihelion_ each 10,500 years--would produce a complete change of climate
only where a country was _partially_ snow-clad; while, whenever a large