158 ISLAND LIFE part i 



a larger proportion would be melted by the longer, though 

 somewhat cooler summer. This would follow because the 

 total amount of sun-heat received during the summer 

 would be the same as before, while it would act on a less 

 quantity of snow ; there would thus be a smaller surface to 

 reflect the heat, and a smaller condensing area to produce 

 fogs, while the diminished intensity of the sun would 

 produce a less dense canopy of clouds, which have been 

 shown to be of prime importance in checking the melting 

 of snow by the sun. We have considered this case, for 

 simplicity of reasoning, on the supposition that all geo- 

 graphical and physical causes remained unchanged. But 

 if an alteration of the climate of the whole north temperate 

 and Arctic zones occurred, as here indicated, this would 

 certainly affect both the winds and currents, in the manner 

 already explained {see p. 142), so as to react upon climate 

 and increase the differences produced by phases of 

 precession. How far that effect would be again increased 

 by corresponding but opposite changes in the southern 

 hemisphere it is impossible to say. It may be that 

 existing geographical and physical conditions are there 

 such potent agents in producing a state of glaciation that 

 no change in the phases of precession would materially 

 affect it. Still, as the climate of the whole southern 

 hemisphere is dominated by the great mass of ice within 

 the Antarctic circle, it seems probable that if the winter 

 were shorter and the summer longer the quantity of ice 

 would slightly diminish ; and this would again react on 

 the northern climate as already fully explained. 



The Essential Principle of Climatal Change Restated, — 

 The preceding discussion has been somewhat lengthy, 

 owing to the varied nature of the facts and arguments 

 adduced, and the extreme complexity of the subject. But 

 if, as I venture to urge, the principle here laid down is a 

 sound one, it will be of the greatest assistance in clearing 

 away some of the many difficulties that beset the whole 

 question of geological climates. This principle is, briefly, 

 that the great features of climate are determined by a 

 combination of causes, of which geographical conditions 

 and the degree of excentricity of the earth's orbit are by 



