334 RECENT CHANGES OF LEVEL 



are now convinced that lie antedated the last or generally accepted glacial period. Many 

 of our existing land animals seem to have survived that great change. It is difficult to see 

 how these things would be possible if the ice action had continued long enough to mould 

 the whole topography of the circumpolar i-egions down to the parallel of 40". 



I have elsewhere discussed the question of the origin of glacial action, and shall not 

 repeat the matter here, except to indicate summarily the opinions to which its consideration 

 seemed to lead. These were briefly as follows : — 



Glacial action, such as is indicated by the records of the last ice period, requires us to 

 suppose an intensification of evaporation over all the oceanic areas between 40° north and 

 south of the Equator. This increased evaporation could only be brought about by an 

 increase in the heat of the surface of the earth causing an enhancement of the suj^ply of 

 vapor for condensation. Of all the possible sources of increased heat the following are the 

 onl_y ones which have been suggested which deserve consideration. 1. Change in the 

 attitude of the land and sea to each other. 2. Change in the temperature of celestial 

 space through which our solar system is moving. 3. Change in the excentricity of the 

 earth's orbit in connection with the precession of the equinoxes. Each of these hypotheses 

 has proved in the end unsatisfactory. We therefore were driven to the h^^pothesis, which 

 is borne out by many considerations, that the sun is a source of var3ring heat ; that it is in 

 fiict a variable star. The sudden increment of its heat by one-half would make the inter- 

 tropical region the seat of intense evaporation, and the cloud-wrapped polar regions the 

 seat of excessive precipitation, while the countcf-trades, or return upper currents of air, 

 would have their carrying power much enhanced by the greater activity of the trade wind 

 system, consequent on the increase in the difference between the temperature of the equa- 

 torial and polar regions. 



Looking upon our sun as a variable star, we get a basis for recurrent change, and can 

 conceive the physical and vital phenomena of our planet moving in accord with the 

 changes in that source of nearly all the ph^ysical and vital energy of the earth's surface. 

 Some evidence has already been obtained of the following glacial periods : — 



1. In the Cambrian, about the zone of Paradoxides Harlani. 



2. At the close of the horizon of the Cincinnati group. 



3. In the time immediately antecedent to tlie heavier coal measures of the carl^oniferous 

 period. 



4. In the trias. Evidence of the occurrence of glaciers in tliis jieriod has Ijeen found in 

 South Africa as well as Europe. 



5. In the tertiary (probably three periods). 



When we consider the extreme liability of such action as glaciation to be destroyed by 

 subsequent agents, it becomes clear that these few periods, determined at the very outset of 

 the research into the history of this agent, must be onl}' a small part of the successive 

 periods of similar change. It is also evident that the recurrence of successive glacial 

 periods must have a most hnportant influence upon the course of life ; forcing great migra- 

 tions, and effecting great changes in the whole character of the conditions of existence. It 

 is not my purpose at this time to consider the evidence which can be Ijrought from other 

 regions to show the immediate value of these changes; and I can only indicate that the hori- 

 zons of all our consjlomerates are horizons where great changes have been effected even in 

 the marine life, which is likely to have been less influenced by the change than the life of 



