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ISLAND LIFE. 
[part 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 Relations.” 
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 
