NEBULAR THEORY. 2G9 



condensed in course of time to a smaller size. At the epoch when 

 Mercury, the planet nearest to the sun, had not yet separated 

 from the solar mass, and when the sun extended as far as the 

 orbit of Mercury, the gigantic central luminary must have ex- 

 erted upon the earth a very different influence from that of the 

 present day, as the solar mass occupied about one fourth of 

 the horizon. As the central body was then much less con- 

 densed than it is at present, a certain part of the sun would 

 emit fewer rays, whether luminous or calorific, than it now does, 

 and these rays would be more uniformly distributed over the 

 earth. 



The torrid zone, all the parts of which, at a given season, re- 

 ceive the solar rays perpendicularly, must have had a wider ex- 

 tent than at present. From this circumstance it follows that 

 light and heat were more uniformly diffused over the earth, and 

 that there could not be a night of six months about the poles. 

 By degrees the mass of the sun, being more and more contracted 

 by a continuous condensation, would be reduced gradually to its 

 present volume. This hypothesis would explain more than one 

 phenomenon of early geological ages, and especially the existence 

 in high northern latitudes of arborescent evergreen plants ; but, 

 in order to admit it, we must believe that a slow and uniformly 

 continuous contraction took place in the mass of the sun, and 

 that in consequence there was a slow and corresponding change 

 in. all the phenomena connected with the solar orb. It is very 

 possible that during the earliest geological phases the sun was 

 larger, and composed of a less dense mass than at present ; but 

 we must not lose sight of the fact that the Miocene epoch was 

 comparatively near to our own time, at least if we appreciate 

 the interval since the Miocene formation by a geological standard. 

 At so late a period as the Miocene epoch it is improbable that 

 the sun had remained in so imperfect a phase, while the crust of 

 the earth and the plants and animals inhabiting it had attained 

 to such a high degree of development. Besides, it has been 

 already noticed that from the Carboniferous to the Cretaceous 

 period no climatal changes can be proved to have taken place, 

 whilst from the Miocene to the beginning of the Quaternary 

 period, during a comparatively short time, a complete alteration 

 took place, and the temperature of the Glacial epoch sank below 



