Fauna and Flora of New Zealand. 85 



These facts, which have been well known for many years, 

 were first brought prominently before geologists by Sir J. 

 Herschel in his address to the Geological Society of London 

 in 1832. From them we know that the earth's orbit has 

 varied in past time, and we infer that these variations must 

 have caused considerable changes in climate. But what these 

 changes were and to what extent they were carried are specu- 

 lative deductions from the laws of physics, and are difficult 

 to verify. The well-known hypothesis of Dr. Croll was the 

 first of these speculations. It is very ingenious and worked 

 out with great skill ; but it must of course be subject to all 

 the complexities and uncertainties in which all meteorological 

 phenomena are involved. 



He maintains that the large quantity of snow that would 

 fall in the winter of each year on that hemisphere whose 

 winter was in aphelion would not be all melted during the 

 hot but short summer that followed, and consequently it 

 would accumulate year by year and bring on a glacial epoch. 

 His reasons for thinking that the whole snowfall would not be 

 melted in summer are : — (1) that, as snow can never rise in 

 temperature above 32° F., the direct radiation from land 

 covered with snow and ice would cool the air and lower the 

 temperature of all surrounding bodies ; (2) that the rays of 

 the sun falling on the snow and ice would to a large extent 

 be reflected back again into space ; and (3) that thick fogs 

 and cloudy skies would effectually' prevent the rays of the 

 sun from reaching the earth. If a reduction of the summer 

 temperature by these means is allowed, he then urges that 

 great changes would take place in the oceanic currents, which 

 would tend still further to lower the temperature of that hemi- 

 sphere on which snow was accumulating. 



I need not allude to the objections that have been urged 

 against this hypothesis. It is sufficient for my purpose to 

 point out that, according to Dr. Croll himself, all depends 

 upon the snow falling on land, for without land there will be 

 no snow to radiate, to reflect, or to form fogs and clouds. 

 Now in the antarctic regions there is no great extent of land 

 that is not already covered with snow. During the long cold 

 winter of a high eccentricity the snow would fall into the sea, 

 would be melted, and work its way towards the equator. 

 Consequently there would be no accumulation, and a high 

 eccentricity would not bring on a glacial epoch in the southern 

 hemisphere. On the contrary, greater cold would probably 

 precipitate the moisture more to the north, and so lessen the 

 snowfall in high antarctic latitudes where alone there is land. 



