200 Proceedings of the Royal Physical Society. 



with snow and ice. But while the chief determining cause 

 of climate has been the relative distribution of land and 

 water, it is impossible to doubt that during periods of 

 high eccentricity of the orbit, the climate must have been 

 modified in a greater or less degree. In our own day the 

 geographical conditions are such that, were eccentricity to 

 attain a high value, the climate of the Pleistocene would 

 be reproduced, and our hemisphere would experience a 

 succession of alternating cold and genial epochs. 



But in earlier stages of the world's history, the geo- 

 graphical conditions were not of a kind to favour the 

 accumulation of vast ice-fields. During a period of extreme 

 eccentricity, there would probably be fluctuations of tem- 

 perature in high latitudes ; but nothing like the glacial 

 and interglacial epochs of the Pleistocene could have 

 occurred. At most, there would be a general lowering of 

 the temperature, sufficient to render the climate of Arctic 

 seas and lands somewhat cooler, and probably to induce 

 the appearance in suitable places of local glaciers ; and, 

 owing to precession of the equinox, these cooler conditions 

 would be followed by a general elevation of the temperature 

 above the normal for the geographical conditions of the 

 period. In Palaeozoic and Mesozoic times, the effects of 

 high eccentricity of tlie orbit appear to have been, in a 

 great measure, neutralised by the geographical conditions, 

 w^ith a possible exception in the Permian period. But in 

 Tertiary times when the land-masses had become more 

 continuous, the cosmical causes of change referred to must 

 have had greater influence. And I cannot help agreeing 

 with Dr Croll that the warm climates of the Arctic regions 

 during that era were, to some extent, the result of high 

 eccentricity. 



In concluding this discussion, I readily admit that our 

 knowledge of geographical evolution is as yet in its infancy. 

 We have still very much to learn, and I shall be the last 

 to dogmatise upon the subject. But I hope I have made 

 it clear that the evidence, so far as it goes, does not 

 justify the confident assertions of Dr Croll's opponents, 

 that his theory is contradicted by what we know of the 



