126 ISLAND LIFE 



5. Changes in the position of the earth's axis of rota- 

 tion. 



6. A variation in the amount of heat radiated by the 

 sun. 



7. A variation in the temperature of space. 



Of the above, causes (1) and (2) are undoubted realities ; 

 but it is now generally admitted that they are utterly in- 

 adequate to produce the observed effects. Causes (5) (6) 

 and (7) are all purely hypothetical, for though such changes 

 may have occurred there is no evidence that they have 

 occurred during geological time ; and it is besides certain 

 that they would not, either singly or combined, be adequate 

 to explain the whole of the phenomena. There remain 

 causes (3) and (4), which have the advantage of being de- 

 monstrated facts, and which are universally admitted to be 

 capable of producing some effect of the nature required, the 

 only question being whether, either alone or in combination, 

 they are adequate to produce all the observed effects. It 

 is therefore to these two causes that we shall confine our 

 inquiry, taking first those astronomical causes whose com- 

 plex and wide reaching effects have been so admirably ex- 

 plained and discussed by Dr. Croll in numerous papers and 

 in his work — " Climate and Time in their Geological 

 Relations." 



Astrooiomical Causes of Changes of Climate. — The earth 

 moves in an elliptical orbit round the sun, which is situated 

 in one of the foci of the ellipse, so that the distance of the 

 sun from us varies during the year to a considerable 

 amount. Strange to say we are now three millions of 

 miles nearer to the sun in winter than in summer, while 

 the reverse is the case in the southern hemisphere ; and 

 this must have some effect in making our northern winters 

 less severe than those of the south temperate zone. But 

 the earth moves more rapidly in tliat part of its orbit which 

 is nearer to the sun, so that our winter is not only milder, 

 but several days shorter, than that of the southern hemi- 

 sphere. The distribution of land and sea and other local 

 causes prevent us from making any accurate estimate of 

 the effects due to these differences ; but there can be no 

 doubt that if our winter were as long as our summer is now 



