34:4: ILLOG 1C AL G KOLOG Y. 



of an intervening period.&quot; And once more, &quot; \ve have alsc 

 in like manner had some success of late years in diminish 

 ing the hiatus which still separates the Cretaceous and 

 Eocene periods in Europe.&quot; To which let ns add that 

 since Hugh Miller penned the passage above quoted, the 

 second of the great gaps he refers to has been very consid 

 erably narrowed by the discovery of strata containing Pa 

 laeozoic genera and Mcsozoic genera intermingled. Never 

 theless, 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 these phenomena, let us 

 glance at the several minor causes that produce breaks in 

 the geological succession of organic forms : taking first, 

 the more general ones which modify climate, and, there 

 fore, 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 arc 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 oc 

 curs at the time of our northern summer ; and the perihe 

 lion 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 equinox 

 es, this state of things will in time be reversed : the Earth 

 will be nearest to the sun during the summer of the north 

 ern hemisphere, and furthest from it during the southern 

 Bummer or northern winter. The period required to com 

 plete the slow movement producing these changes, is nearly 

 2G,000 years ; and were there no modifying process, the 

 two hemispheres would alternately experience this coinci 

 dence of summer with the least distance from the sun, dur 



