88 Dr. J. Croll on the Physical Theory 



concerned. Although, it could be satisfactorily shown, for 

 example, and this has certainly not yet been done, that during 

 some past geological age, such as the Miocene, the Eocene, 

 or the Cretaceous, the climate was throughout uniformly warm 

 or subtropical, this would not prove that the theory was wrong, 

 unless it could at the same time be shown that the necessary 

 conditions demanded by the theory did then exist. But in- 

 stead of this supposed condition of climate during Secondary 

 and Tertiary periods being inconsistent with my theory, the 

 fact is, as we shall see by and by_, that this theory affords 

 the only rational explanation of such a state of things which 

 has yet been given. 



2nd. The theory is not that a high state of eccentricity will 

 necessarily produce a glacial epoch. No misapprehension 

 has been more widespread or more difficult to remove than 

 this. From the very commencement I have maintained that 

 no amount of eccentricity, however great, could produce a 

 glacial condition of things; that the Glacial Epoch was the 

 result, not of a high state of eccentricity, but of a combina- 

 tion of Physical Agencies, brought into operation by means 

 of this high state *. As an example of this misapprehension, 

 how frequently has the present condition of the planet Mars 

 been adduced as evidence against the theory. The eccentri- 

 city of Mars's orbit is at present greater than that of the 

 Earth's even when at its superior limit; and its southern 

 winter solstice is not far removed from aphelion. It is there- 

 fore maintained that, if my theory of the cause of the glacial 

 epoch be correct, the southern hemisphere of Mars ought to 

 be under a glacial condition, and the northern enjoying a 

 perpetual spring — and this, as is well known, is not the 

 case. Here it is assumed that, according to the theory, eccen- 

 tricity alone ought to produce a glacial epoch, irrespective of 

 the necessary physical conditions. We know with certainty 

 that those physical conditions which, according to the theory, 

 were the direct cause of the glacial epoch on our globe, can- 

 not possibly exist on the planet Mars f. Just take one ex- 

 ample : either the properties of water on the planet Mars or 

 the conditions of its atmosphere must be totally different from 

 those of our earth; for were our earth removed to Mars's dis- 

 tance from the sun, our seas would soon become solid ice and 

 we could have neither snow nor rain, ocean-currents, nor any 

 of the necessary conditions for secular change of climate. 

 This is doubtless not the present state of Mars; but the reason 

 of this can only be that the physical and meteorological con- 



* For this reason I prefer to term the theory the Physical Theory 

 rather than the Eccentricity Theory, as it has been called by some writers. 

 t See < Climate and Time.' p. 79. 



