THE ORIGIN OF THE FAUNA AND FLORA OF NEW ZEALAND. 255 
the orbit gets flatter the position of the sun continually recedes 
from the centre; or, in other words, the orbit gets more and 
more eccentric. As the annual amount of heat received by the 
earth from the sun varies inversely as the length of the minor 
axis, it follows that the greater the eccentricity the greater is the 
total amount of heat received from the sun. But when the 
eccentricity is great the earth is much nearer the sun in perihe- 
lion, and further away from it in aphelion, than when the eccen- 
tricity is small. Consequently that hemisphere of the earth 
which has its winter in aphelion and its summer in perihelion 
during a time of great eccentricity, will have its seasons exag- 
gerated ; a long and very cold winter being followed by a short 
but very hot summer; while the other hemisphere will have a 
short warm winter, followed by alongandcool summer. Owing 
however to the combined action of precession of the equinoxes 
and revolution ot the apsides, the hemisphere which has its win- 
‘ter in aphelion is changed every 10,500 years; and as a period 
of great eccentricity will last longer than this, these alternations 
of climate will recur perhaps three or four times before the eccen- 
tricity is greatly diminished. 
These facts, which have been well known for many years, 
were first brought prominently before geologists by Sir J. Her- 
schel 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 speculative deductions 
trom the laws of physics, and are difficult to verify. The well- 
known hypothesis of Dr. Croll was the first of these specula- 
tions. It is very ingenious and worked out with great skill; but it 
must of course be subject to all the complexities and uncer- 
tainties 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 snow-fall 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 wuuld effectually prevent the 
rays of the sun from reaching the earth. It a reduction of the 
summer temperature by these means is allowed, he then argues 
that great changes would take place in the oceanic currents, 
which would tend still further to lower the temperature of that 
hemisphere 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 
