G. W. Biilman — Revised Theory of Olaciation. 267 



bution as witnesses of the truth of the Revised Astronomical Theory. 

 The facts, however, seem to me rather to testify against it. The 

 argument is briefly thus : 



It has long been known that certain temperate forms of plants in 

 regions far removed from each other and on opposite sides of the 

 equator, exhibit a remarkable similarity. 



So great is the likeness that it is supposed that those in the south 

 must have migrated from the north, or vice versa. They must, 

 then, liave crossed the equator, and that they did so is a proof that 

 equatorial regions were at one time cooled down sufficiently to 

 permit of the existence in them of these temperate forms of 

 vegetation. And the Astronomical Theory requires that equatorial 

 regions in the glaciated hemisphere should be thus cooled down. 

 Thus the botanical facts seems to confirm the Asti-onomical Theory. 



But, although the lengthening of one season will reduce its average 

 temperature, that of the other will be increased by the corresponding 

 shortening. And if the present heat of the tropics during summer 

 in our northern hemisphere would be fatal to temperate plants, much 

 more would it be so during glaciation when the summer supply 

 of heat was concentrated into 166 days. A calculation of what 

 the temperature of the glacial summer would be from Sir R. Ball's 

 figures will perhaps make this more obvious. The mean daily heat 

 during the glacial summer is given as 1*38, and at present as 1"24, 

 which gives a diiference of -14. Now if •! of a heat unit corre- 

 sponds to 30°, -14 will correspond to 42.° Taking the present 

 average temperature of our equatorial regions as 80°, this gives 122° 

 as that of the glacial summer ! 



The botanical facts, then, seem to testify that if a glacial period 

 helped the plants over the equator, glaciation was n9t brought about 

 according to the Revised Astronomical Theory. 



In seeking in the stratified rocks for evidence of the numerous 

 glacial periods which must, according to the Astronomical Theory, 

 have occurred in the past. Sir R. Ball's method of interpreting the 

 record has at least the merit of novelty. Dr. Croll, and other 

 geologists, when in search of such evidence have weighted them- 

 selves with the necessity of finding scratched and polished boulders, 

 erratics, and other deposits analogous to those formed during the 

 latest ice age. Sir R. Ball has rid himself of this impediment, and 

 finds in the ordinary alternation of stratified beds a record of alter- 

 nate mild and glacial epochs. 



The scarcity of evidence of former Ice Ages, sufficient to satisfy 

 the average geologist, is well known, and has hitherto formed one 

 of the most — if not the most — serious stumbling blocks in the way 

 of the Astronomical Theory of Glaciation. And the Revised Theory 

 sufi'ers more in this respect than did that of Dr. Croll. For accord- 

 ing to the former glaciation follows as a necessary consequence, 

 whenever the requisite astronomical conditions occur, viz. whenever 

 by combination of high eccentricity and the approach of the line of 

 equinoxes to a position perpendicular to the major axis of the earth's 

 orbit, the difference in length between the two seasons becomes 



