II THE PROGRESS OF SCIENCE 113 



spheres of our globe, have brought astronomy into 

 intimate relation with geology. Geology, in fact, 

 proves that, in the course of the past history of 

 the earth, the climatic conditions of the same 

 region have been widely different, and seeks the 

 explanation of this important truth from the sister 

 sciences. The facts that, in the middle of the 

 Tertiary epoch, evergreen trees abounded within 

 the arctic circle ; and that, in the long subse- 

 quent Quaternary epoch, an arctic climate, with 

 its accompaniment of gigantic glaciers, obtained 

 in the northern hemisphere, as far south as 

 Switzerland and Central France, are as well 

 established as any truths of science. But, whether 

 the explanation of these extreme variations in the 

 mean temperature of a great part of the northern 

 hemisphere is to be sought in the concomitant 

 changes in the distribution of land and water 

 surfaces of which geology affords evidence, or in 

 astronomical conditions, such as those to which I 

 have referred, is a question which must await its 

 answer from the science of the future. 



Turning now to the great steps in that vast 

 progress which the biological sciences have made 

 since 1837, we are met, on the threshold of our 

 epoch, with perhaps the greatest of all namely, 

 the promulgation by Schwann, in 1839, of the 

 generalisation known as the " cell theory," the 

 application and extension of which by a host 

 of subsequent investigators has revolutionised 



VOL. I I 



