MODIFICATION OF THEORY EXAMINED. 137 



climate much milder than the present, the probability 

 is that they would be classified as preglacial. I fully 

 agree with Prof. J. Geikie, that many of those plants 

 and animals of a southern type which have been 

 regarded as preglacial are in reality of intergiacial age. 

 Objection as to the Number of Intergiacial Periods. 

 — It has been urged as an objection to the Physical 

 theory of the Glacial Epoch, that, according to it, there 

 ought to have been more intergiacial periods than we 

 have direct evidence of having actually occurred. I 

 am doubtful as to the force of this objection. I do 

 not think that there could have been more than about 

 five well-marked intergiacial periods during the entire 

 Glacial Epoch ; three probably during the former half 

 of the epoch, and certainly not more than two during 

 the latter half. There would be a large interval 

 between the two maxima of eccentricity of 100,000 

 and 200,000 years ago, when the alternations of 

 climate would be comparatively moderate in extent. 

 Besides, it is not correct to assume, as is generally 

 done, that the interval between two consecutive inter- 

 giacial periods is only 21,000 years ; for the mean rate 

 of motion of the perihelion during the Glacial Epoch 

 was considerably less than has been assumed. It will 

 be seen from the Table of the Longitude of the 

 Perihelion, given in ' Climate and Time,' p. 320, that 

 it has taken the perihelion 231,000 years to make one 

 complete revolution. If, therefore, we assume, what of 

 course is not certain, that the mean rate of precession 

 during the Glacial Epoch was the same as the present, 

 then the rate of precession to that of the perihelion's 

 motion would, in this case, be as nine to one. The 

 equinoxial point will take 25,811 years to make one 

 revolution ; but as the perihelion moves in the opposite 

 direction, it will reduce the time taken by the point in 



