408 EXISTENCE AND DISTRIBUTION OF MARINE ORGANISMS. 



between the earth and the other members of the solar system were in 

 past times the same as we now find them. The variation in the astro- 

 nomical elements of the globe is so small that they are regarded as 

 stable for the period covered by history, but this variation assumes 

 great importance when the periods represented by geology are brought 

 into consideration. 



Lord Kelvin says the nebular theory of the evolution of the solar sys- 

 tem, ''founded on the natural history of the stellar universe, as observed 

 by the elder Herschel and completed in details by tbe profound dynam- 

 ical judgment and imaginative genius of Laplace, seems converted by 

 thermodynamics into a necessary truth if we make no other uncertain 

 assumption than that tbe materials at present constituting the dead 

 matter of the solar system have existed under the laws of dead matter 

 for a hundred million years." ' A large sun during the early stages of 

 the earth's history is therefore a necessary result of what is believed to 

 have been the genesis of our system. Tbe earth is an extremely small 

 fragment thrown off from the central sun at one of its periods of con- 

 densation, and by reason of its small dimensions its stellar phase would 

 be comparatively short. On the other hand, the enormous mass of the 

 sun would cool much more slowly, and its gradual contraction would 

 provide an amount of energy sufficient to make good all that lost in 

 radiation. 



We may well suppose that when the sun had a diameter little less 

 than the diameter of the orbit of Mercury the precipitation of water,, 

 geological and life phenomena, commenced on our earth. 2 Such a 

 nebulous sun would radiate for each unit of its surface less heat and 

 light than the sun at present, but the total amount of radiant energy 

 received by the earth might be greater than that received at present,, 

 and would be very differently distributed over the earth's surface.. 

 The Torrid Zone would be extended on either side of the equator to 

 the forty-seventh parallel of latitude. The seasonal effects produced 

 by the inclination of the earth's axis to the ecliptic would be annulled. 

 There would be suppression of a twenty-four-hour night about the 

 poles at any time of the year. A cone of effective solar rays would 

 graze the earth along a small circle of the sphere; at the solstice the 

 rays of light would touch one pole and envelop the other to the forty- 

 third parallel of latitude, so that at this position of the earth four 

 degrees of latitude — those between 43° and 47° — would have at the 

 the same time a twenty-four-hour day and some portion of tlie sum 

 overhead at noon. A sun the angular diameter of which is equal to 

 twice the obliquity of the ecliptic, i. e., 47°, would thus produce at the 

 poles during the whole year an insolation 18 per cent greater than at 



1 Kelvin. Popular Lectures and Addresses, Vol. I ; pages 421-422. London, 1891. 

 " Les geolognes pourront trouver, dans le diametre considerable de la masse solairo 

 a ces epoques, l'explication de IVgalite <lc climat dont paralt avoir joui la terre 

 jusqu'au commencement do l'epoque actueile." (Wolf. Les Hypotheses cosmogoni- 

 que.s, page 32; Paris, 188G). 



