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 remainsof a fauna of an intervening period.??? And once more, he says, ???wehave 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 phiage above quoted, the second of the great gaps he refers to has been very considerably narrowed by the discoveryof strata containing Pal??ozoic genera and Mesozoic genera intermingled. Nevertheless, the occurrence oftwo great revolutions in the

Earth???s Flora and Fauna appears still to be held by many; and geologic nomenclature habitually hiumes 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 onewhich has not, we believe, been named by writers on thesubject. 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 with relative nearness to the sun, during a period of 13,000 years. But there is also a still slower change in the direction of the axis major of the Earth???s orbit; from which it results that the alternation we have described is completed in about 21,000 years. That is to say, if at a given time the Earth is nearest to the sun at our mid-summer, and furthest from the sun at our mid-winter; then, in 10,500 years afterwards, it will be furthest from the sun at our mid-summer, and nearest at our mid-winter. Now the difference between the distances from the sun at the two extremes of this alternation, amounts to one-thirtieth; and hence, the difference between the quantities of heat received from the sun on a summer???s day under these opposite conditions amounts to one-fifteenth. Estimating this, not with reference to the zero of our thermometers, but with reference to the temperature of the celestial spaces, Sir John Herschel calculates ???23?? Fahrenheit, as the least variation of temperature under such circumstances which can reasonably be .