1874.] Climate of the Glacial Period. 433 
summer. The fallacy of the argument can, however, be 
best shown by considering what would be the effect of 
diminishing the obliquity to zero. When the direction of 
the axis of rotation of the earth became perpendicular to 
the plane of its orbit, the difference of the seasons of the 
year would disappear and perpetual spring would reign in 
the arctic regions. All over the globe there would be 
twelve hours night and twelve hours day, and no amount of 
ellipticity of the orbit could have any effect in lengthening 
the nights and days. Every step in the diminution of the 
obliquity of the ecliptic would be an approach towards this 
state of perpetual equinox, and tend more or less to equalise 
the seasons. The theory of Mr. Croll is based on an 
assumed exaggeration, by increased eccentricity of the 
orbit, of the effects of the present obliquity of the ecliptic, 
and it is startling to find it urged that a decrease in that 
obliquity would increase the results. 
Having thus shown that the foundations of the theory 
present many points of weakness, I shall next take into con- 
sideration the question of how far it is in harmony with the 
geological facts sought to be explained by it. One of the points 
insisted upon by Mr. Croll, and which is stated to be in ac- 
cordance with the facts known to geologists, is that during 
the greatest eccentricity of the orbit periods of glaciation 
would alternate with others of great warmth. Whilst one 
hemisphere was undergoing the extreme rigour of a glacial 
period, the other would rejoice in a ‘‘ perpetual summer.” 
And, owing to the precession of the equinoxes by which 
there is a complete revolution of the equinoétial point in 
21,000 years, in half that time the hemisphere that had its 
winter in aphelion would slowly change until it had it in 
perihelion. The ice that had been heaped up at one pole 
would melt away and be piled up at the other. And as the 
last greatest period of elllipticity occupied, according to 
Mr. Croll’s laborious calculations, about 160,000 years, 
there would during that time be several complete revolutions 
of the precession of the equinoxes, so that each hemisphere 
would have alternately several glacial periods and several 
warm periods. 
To prevent misconception I shall give Mr. Croll’s opinion 
on this question in his own words. He says:—‘It is 
physically impossible that we can have a cold and arétic 
condition of climate on the one hemisphere, resulting from 
a great increase of eccentricity, without at the same time 
having a warm, equable, if not an almost tropical, condition 
of climate prevailing on the other hemisphere.” ‘If the 
