143 
ISLAND LIFE. 
[part I. 
glaciation when moraines were left in the valleys ; several succes- 
sive periods of submergence and elevation, the later ones becom- 
ing less and less in amount, as indicated by the raised beaches 
slightly elevated above our present coast line ; and lastly, the 
occurrence in the same deposits of animal remains indicating 
both a warm and a cold climate, and especially the existence 
of the hippopotamus in Yorkshire soon after the period of 
extreme glaciation. 
But although the evidence of some alternations of climate 
seems indisputable, and no suggestion of any adequate cause 
for them other than the alternating phases of precession during 
high excentricity has been made, it by no means follows that 
these changes were always very great— that is to say, that the 
ice completely disappeared and a warm climate prevailed 
throughout the whole year. It is quite evident that during 
the height of the glacial epoch there was a combination of 
causes at work which led to a large portion of North-western 
Europe and Eastern America being buried in ice to a greater 
extent even than Greenland is now, since it certainly extended 
beyond the land and filled up all the shallow seas between 
our islands and Scandinavia. Among these causes we must 
reckon a diminution of the force of the Gulf Stream, or its being 
diverted from the north-western coasts of Europe ; and what w*e 
have to consider is, whether the alteration from a long cold 
winter and short hot summer, to a short mild winter and long 
cool summer would greatly affect the amount of ice if the 
ocean currents remained the same. The force of these currents 
are, it is true, by our hypothesis, modified by the increase 
or diminution of the ice in the two hemispheres alternately, 
and they then react upon climate; but they cannot be thus 
changed till after the ice-accumulation has been considerably 
affected by other causes. Their direction may, indeed, be 
greatly changed by slight alterations in the outline of the land, 
while they may be barred out altogether by other alterations 
of not very great amount ; but such changes as these have no 
relation to the alteration of climates caused by the changing 
phases of precession. 
Now, the existence at the present time of an ice- clad 
