THE CAUSE OF THE GLACIAL PEBIOD. 529 



knowledge of the whole subject is in an extremely unsatis- 

 factory condition ; and in this, as in other things, the first 

 requisite of progress is to squarely face the extent of our 

 ignorance upon the question. The causes with which the 

 glacialist deals are extremely complicated, and yet they are 

 of such a nature as to invite investigation, and to hold out 

 the hope of increasing success in mastering the problem. 

 There is opportunity yet for some Newton or Darwin to 

 come into the field and discover a clew with which success- 

 fully to solve the complicated problem which has so far baf- 

 fled us. To the genuine investigator it is a source of inspi- 

 ration rather than of depression to have such an untrodden 

 field before him. 



Conclusion. — Geology is pre-eminently a terrestrial sci- 

 ence, and there is danger of a misdirection of effort when 

 the geologist forms an alliance with the astronomer. Astro- 

 nomical data are so largely theoretical, and the quantities 

 which the astronomer multiplies are often so nearly infinitesi- 

 mal, that quantitative error is in peculiar danger of becom- 

 ing enormous in large calculations. Hence, we can not count 

 it altogether an advantage that astronomical speculation has 

 been so rife during the past few years in determining the 

 causes and the chronology of the Glacial period. 



Of the various cosmical theories to account for the Gla- 

 cial period, that of Mr. Croll is by far the most plausible and 

 interesting. It must be admitted that his data concerning 

 the various distances at which the earth is found from the 

 sun during the winters of different periods, and concerning 

 the periodical variations in the length of the winters, rest 

 upon well-ascertained facts. It is no doubt true that about 

 one hundred thousand years ago the winters were at times 

 several days longer than now, and the northern hemisphere 

 was receiving daily considerably less heat than now, since it 

 was several millions of miles farther away from the sun. 



But the distribution of the earth's heat by winds and 

 oceanic currents is a subject concerning which much less is 

 known. The phenomena presented in a hot-house are puz- 



