54 FORMATION OF THE EARTH 



They are dominated by two outstanding facts : First, that as 

 far as our observation can be extended, that is to say during 

 a period of nearly twenty million years, the polar caps retained 

 a climate resembling that of the Mediterranean to-day ; for 

 this to be the case it would be essential that they should have 

 been almost permanently illuminated by the sun. Secondly, 

 the torrid zone, on the other hand, had remained constant in 

 climate ever since the origin of life, but it had gradually 

 decreased in extent, and there had been a very slow but con- 

 tinuous cooling of the polar regions, whose climate, having 

 become rigorous and glacial, was preserved in regions 

 corresponding to our present temperate zone. This we can 

 explain only in one way, by admitting a progressive shrinking 

 of the solar disc, and it is therefore to this dominating cause 

 that those other and secondary conditions appertain that have 

 given to the Pleistocene Period the variety of characters 

 peculiar to it. 



It is therefore the sun which has guided the evolution of the 

 earth. The sun has shown himself to be the great artist who 

 can repeat over and over again the forms of living creatures, 

 and who is himself the creator of every variety of living form. 

 The periodic alterations in the earth's orbit and the 

 inclination of the axis to the ecliptic, mentioned at the 

 beginning of this chapter, have undoubtedly played their part 

 here, too, so that if geologists and astronomers could be 

 induced to co-operate in their researches into these questions, 

 it might be possible, by exhaustive discussion of the testimony 

 gathered by each, to find some method of explaining the 

 principal geological events and arriving at an accurate 

 chronology. The calculations of astronomers themselves do 

 not take into consideration all the facts of the problem, for 

 they have specifically studied the consequences of those events 

 that, on the supposition of stationary stars, relate to the 

 influences they exercise upon one another. Everything relating 

 to modifications in the form and constitution of the earth 

 escapes their calculations, though these modifications must 

 have been relatively greater than those acting on the sun, 

 owing to the smaller size of the earth. These modifications may 

 have intervened to increase the inclination of the earth's axis 

 of rotation to the ecliptic, or to have rendered it perpendicular 

 to the same without changing the geographical position of the 



