122 



ISLAND LIFE. 



[PAET I. 



4. Changes in the distribution of land and water. 



5. Changes in the position of the earth's axis of rotation. 



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 inadequate 

 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 demonstrated facts, and which are uni- 

 versally 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 complex 

 and wide reaching effects have been so admirably explained and 

 discussed by Dr. Croll in numerous papers and in his work — 

 " Climate and Time in their Geological Eelations." 



Astronomical 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 that 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 

 hemisphere. 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 now as long as our summer, and we were also 

 three million miles further from the sun at the former period, a 

 very decided difference of climate would result — our winter 



