CONTINUITY. 207 



the absence of older and appearance of newer allied or unallied 

 genera.' 



Indications of the connection between cosmical studies 

 and geological researches are dawning on us : there is, for 

 instance, some reason to believe that we can trace many 

 geological phenomena to our varying path of rotation round 

 the sun ; thus, more than thirty years ago Sir J. Herschel 

 proposed an explanation of the changes of climate on the 

 earth's surface as evidenced by geological phenomena, founded 

 on the changes of eccentricity in the earth's orbit. 



He said he had entered on the subject ' impressed with 

 the magnificence of that view of geological revolutions which 

 regards them rather as regular and necessary effects of great 

 and general causes, than as resulting from a series of convul- 

 sions and catastrophes regulated by no laws and reducible to 

 no fixed principles.' 



As the mean distance of the earth from the sun is nearly 

 invariable, it would seem at first sight that the mean annual 

 supply of light and heat received by the earth would also be 

 invariable; but according to his calculations it is inversely 

 proportional to the minor axis of the orbit : this would give 

 less heat when the eccentricity of the earth's orbit is approach- 

 ing towards or at its minimum. Mr. Croll has recently shown 

 reason to believe that the climate, at all events in the circum- 

 polar and temperate zones of the earth, would depend, in 

 some degree, on whether the winter of a given region occurred 

 when the earth at its period of greatest eccentricity was in 

 aphelion or perihelion if the former, the annual average of 

 temperature would be lower, if the latter, it would be higher 

 than when the eccentricity of the earth's orbit were less, or 

 approached more nearly to a circle. He calculates the dif- 

 ference in the amount of heat at the period of maximum 

 eccentricity of the earth's orbit to be at nineteen to twenty-six, 

 according as the winter would take place when the earth was 

 in aphelion or in perihelion. His reason may be briefly stated 

 thus: assuming the mean annual heat to be the same, what- 

 ever the eccentricity of orbit, yet if the extremes of heat and 

 cold in summer and winter be greater, a colder climate will 

 prevail, for there will be more snow and ice accumulated in 



