A HALF-CENTURY OF SCIENCE. 77 



of temperature lasting for no less than twenty-one thousand years. 

 This explains the fact that, as Morlot showed in 1854, the glacial de- 

 posits of Switzerland, and, as we now know, those of Scotland, are not 

 a single uniform layer, but a succession of strata indicating very dif- 

 ferent conditions. I agree also with Croll and Geikie in thinking that 

 these considerations explain the apparent anomaly of the coexistence 

 in the same gravels of arctic and tropical animals ; the former having 

 lived in the cold, while the latter flourished in the hot, periods. 



It is, I think, now well established that man inhabited Europe dur- 

 ing the milder periods of the glacial epoch. Some high authorities, 

 indeed, consider that we have evidence of his presence in pre-glacial 

 and even in Miocene times, but I confess that I am not satisfied on 

 this point. Even the more recent period carries back the record of 

 man's existence to a distance so great as altogether to change our 

 views of ancient history. Nor is it only as regards the antiquity and 

 material condition of man in prehistoric times that great progress has 

 been made. If time permitted, I should have been glad to dwell on 

 the origin and development of language, of custom, and of law. On all 

 of these the comparison of the various lower races, still inhabiting so 

 large a portion of the earth's surface, has thrown much light ; while 

 even in the most cultivated nations we find survivals, curious fancies, 

 and lingering ideas, the. fossil remains, as it were, of former customs 

 and religions imbedded in our modern civilization, like the relics of ex- 

 tinct animals in the crust of the earth. 



In geology the formation of our Association coincided with the ap- 

 pearance of Lyell's " Principles of Geology," the first volume of which 

 was published in 1830, and the second in 1832. At that time the 

 received opinion was that the phenomena of geology could only be 

 explained by violent periodical convulsions, and a high intensity of 

 terrestrial energy culminating in repeated catastrophes. Hutton and 

 Playfair had indeed maintained that such causes as those now in opera- 

 tion would, if only time enough were allowed, account for the geologi- 

 cal structure of the earth ; nevertheless, the oj)posite view generally 

 prevailed, until Lyell, with rare sagacity and great eloquence, with a 

 wealth of illustration and most powerful reasoning, convinced geolo- 

 gists that the forces now in action are powerful enough, if only time 

 be given, to produce results quite as stupendous as those which science 

 records. 



As regards stratigraphical geology, at the time of the first meeting 

 of the British Association at York, the strata between the carbonifer- 

 ous limestone and the chalk had been mainly reduced to order and 

 classified, chiefly through the labors of William Smith. But the clas- 

 sification of all the strata lying above the chalk and below the carbon- 

 iferous limestone respectively, remained in a state of the greatest con- 

 fusion. The year 1831 marks the period of the commencement of the 

 joint labors of Sedgwick and Murchison, which resulted in the estab- 



