between the fossils of the Lias and those of the Magnesian Limestone,
is supplied in Germany by the rich fauna and flora of the Muschelkalk,
Keuper, and Bunter Sandstein, which we know to be of a date precisely
intermediate." Again he remarks that "until lately the fossils of the
coal-measures were separated from those of the antecedent Silurian group
by a very abrupt and decided line of demarcation; but recent discoveries
have brought to light in Devonshire, Belgium, the Eifel, and Westphalia,
the remains of a fauna of an intervening period." And once more, he
says, "we have also in like manner had some success of late years in
diminishing the hiatus which still separates the Cretaceous and Eocene
periods in Europe." To which let us add that, since Hugh Miller penned
the passage above quoted, the second of the great gaps he refers to has
been very considerably narrowed by the discovery of strata containing
Palaeozoic genera and Mesozoic genera intermingled. Nevertheless, the
occurrence of two great revolutions in the Earth's Flora and Fauna
appears still to be held by many; and geologic nomenclature habitually
assumes it.

Before seeking a solution of the problem thus raised, let us glance at
the several minor causes which produce breaks in the geological
succession of organic forms; taking first, the more general ones which
modify climate, and, therefore, the distribution of life. Among these
may be noted one which has not, we believe, been named by writers on the
subject. We mean that resulting from a certain slow astronomic rhythm,
by which the northern and southern hemispheres are alternately subject
to greater extremes of temperature. In consequence of the slight
ellipticity of its orbit, the Earth's distance from the sun varies to
the extent of some 3,000,000 of miles. At present, the aphelion occurs
at the time of our northern summer; and the perihelion during the summer
of the southern hemisphere. In consequence, however, of that slow
movement of the Earth's axis which produces the precession of the
equinoxes, this state of things will in time be reversed: the Earth
will be nearest to the sun during the summer of the northern hemisphere,
and furthest from it during the southern summer or northern winter. The
period required to complete the slow movement producing these changes,
is nearly 26,000 years; and were there no modifying process, the two
hemispheres would alternately experience this coincidence of summer