374 Dr. J. Croll on the Physical Theory 



When we pass beyond the limits reached by the ice-sheets 

 of the Glacial Epoch we may expect, of course, to find the 

 remains of many of the plants and animals which lived during 

 the earlier interglacial periods. But here, again, we encounter 

 another difficulty; for we have in this case seldom any means 

 of determining the age to which these remains belong. Unless 

 in relation to overlying and underlying boulder-clays, there 

 seems in many cases no way of knowing to what interglacial 

 period they ought to be assigned ; or, in fact, whether they 

 are really interglacial or not. If the remains in question 

 indicated a condition of 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 interglacial age. 



Objection as to the Number of Interglacial 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 interglacial 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 interglacial periods during 

 the entire Glacial Epoch ; three probably during the former 

 half of Ihe 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 interglacial 

 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 9 to 1. 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 passing from perihelion round to perihelion to 23,2oO 

 years, which will represent the mean interval between two 

 consecutive interglacial periods. But as the motion of the 

 perihelion was very irregular, the length of the interval be- 

 tween the periods would of course differ considerably. 



