7 
An ASTRONOMICAL STANDPOINT, 
If then it is possible to assign the causes of this glacial period, and 
to find among them one of an astronomical nature, which admits 
of calculation backwards in a definite number of years, we shall 
have very certain data, on which to assign an approximate date, 
if not for the first appearance, at any rate for the first certain 
knowledge of a wide distribution of the human species on the 
earth. Croll’s celebrated theory professes to give us such an 
astronomical standpoint. It is an undoubted fact that the earth’s 
orbit is not circular but elliptic, and that the eccentricity varies 
through long periods, making the aphelion portion of the orbit 
sometimes a little different from that of perihelion, and sometimes 
greatly in excess of it. At present the North Pole is turned 
away from the sun in perihelion so that in winter we are nearer 
the sun than in summer, and winter is shorter than summer. 
With the present nearly circular orbit the difference is not great, 
and probably insufficient to cause any very marked effect. But 
with a high eccentricity, when, with winter in aphelion in the 
northern hemisphere, the sun was six and a half millions of miles 
further from the earth than at present, and the winter was 283 
days longer than the summer, it is scarcely conceivable that, if 
other conditions had induced a glacial period, its effects should not 
have been greatly intensified by such an astronomical cause,which 
existed more or less for a period of 160,000 years. It is true 
that the summers would then be hotter in proportion as the 
winters were colder, the absolute amount of heat received from 
the sun being uniform for the shorter and nearer, and for the 
longer and remoter divisions of the year. But the effects on 
climate would be very different, for the greater summer heat 
would be largely exhausted in radiation from a white surface, in 
melting masses of snow and ice, and in evaporation; while it 
would be intercepted by fogs and clouds, and diffused over the 
whole globe by aerial and oceanic currents. The increased 
winter’s cold, on the other hand, would be fixed and stored up to a 
great extent by the conversion of water in its liquid forms of rain 
and sea, into the solid forms of snow and ice. Itis difficult there- 
fore to doubt that Croll’s astronomical theory has been a vera 
causa of the Glacial Period. 
PaLzouitHic Man. 
_ Those who wish to pursue this interesting subject further will 
_ find it fully discussed in Wallace’s Jsland Life, where it is shown 
that such geographical changes as have certainly occurred in 
-eocene and miocene times, and in all probability in pliocene and 
quaternary periods, would account both for the former mild 
climates in arctic regions, and for the setting in of the glacial 
period with its vicissitudes of greatest cold and inter-glacial periods, 
