invariably accompany each other.* The chances of migration 
would then be exceedingly small. 
28. By the time Mr. Darwin wrote his fifth edition of the 
Origin of Species, Dr. James Croll had made public his hypo- 
thesis of the caicse of the Glacial epoch. Mr. Darwin embraced 
the hypothesis,, which is that either the Northern or the Southern 
Hemisphere, having its winter solstice in aphelion at a period 
of great eccentricity, the hemisphere in that position, in conse- 
quence of the earth^s greater distance from the sun, would be 
in a condition of glaciation. If so, the glaciation of the hemi- 
spheres would alternate during successive periods of 10,500 
years. In the fourth edition, Mr. Darwin had spoken of the 
cold of both hemispheres being simultaneous, and then felt 
the difficulty of understanding how the tropical productions 
could have escaped annihilation. The difficulty was removed 
if Dr. Crolhs theory were correct ; but, unconnected with the 
present question, I have given my reasons for believing that it 
is not correct, and Professor Birks f in this room corroborated 
my views. According to the hypothesis, the Southern Hemi- 
sphere ought now to be in a state of glaciation (if the eccen- 
tricity were sufficient), for the Southern Hemisphere has at 
present its winter solstice in aphelion, but the eccentricity is 
only 3,000,000 of miles. When the Northern Hemisphere was 
supposed to have had its last glaciation the eccentricity was 
10,500,000. The question is often asked if the Southern 
Hemisphere is not a nearer approach to a glacial condition 
than is the Northern ? The answer is in the affirmative, but 
not because of the three millions eccentricity, but on account 
of its larger volume of water. The mean annual temperature 
of the Southern Hemisphere is lower than that of the Northern, 
but the mean winter temperature is higher by 5°. J It is not 
winter severity, but summer coolness, that makes the south 
what it is ; the mean summer heat does not reach 60°, whilst 
that of the north is above 70° ; an increased eccentricity 
would intensify the cold in winter (if it had any effect at all) 
and increase the temperature in summer, and so produce a 
climate more like that of the present Northern Hemisphere, 
which is not now under glacial conditions. Mr. Joseph 
Murphy well remarks that ‘^an examination of the facts of 
physical geography shows that the extent of glaciation depends 
* Climate and Time, p. 390. 
t “ Modern Geogonies examined in their bearings on the Antiquity of 
Man,” Transactions of the Victoria Institute, vol. xiii. p. 16. 
X Distribution of Heat on the Surface of the Professor Dove, 
July 28th, 1852, 
