ANNIVERSARY ADDRESS OF THE PRESIDENT. XXXIX 



species that become extinct to amount to 16 per cent. Let us further 

 suppose another change in the neighbourhood of the shallow sea, an 

 elevation of the land to such an extent that perpetual snow would 

 rest on mountains and form glaciers ; a change of temperature in 

 the atmosphere and in the adjoining sea would then take place, and 

 such moUusks as were fitted to live only in a mild temperature would 

 perish and become extinct ; the same might happen from changes 

 in the sea-bottom by the influx of detritus from the land ; and new 

 species adapted to the altered conditions would prevail. Let us sup- 

 pose that the number of species that become extinct by this second 

 change of conditions, also amounted to 16 per cent. ; this last 

 state of things continuing unchanged for a long period, a new ex- 

 ertion of subterranean force raises up the bed of the shallow sea into 

 dry land, and fissures exhibit sections of its structure. A future 

 geologist examines the fossiliferous beds, and he finds a lower series 

 with not more than 68 per cent, of the species living in the adjoining 

 sea; a series of beds above these with about 84 per cent., and these 

 last capped by a series in which all the mollusca are species then 

 existing in the neighbouring sea. He thus finds a series of Older 

 Pliocene, Newer Pliocene, and Post-Pliocene deposits ; for he has no 

 proof that the species he considers extinct are living in any other 

 part of the earth. 



But while these changes have been going on throughout a long 

 period in this part of the ocean (a), the other part of the ocean (b) has 

 continued without any other change than a greater accumulation of 

 sedimentary deposits on its bottom. But that sea-bottom is in process 

 of time elevated at once into dry land, and when examined by a future 

 geologist, he finds the strata containing littoral species of shells, those 

 that live in zones of moderate depth and some that inhabit deep 

 water, some peculiar to sandy some to rocky bottoms. He goes out 

 with his dredge into the neighbouring seas, and makes a large collec- 

 tion of shells from various depths ; and on comparing the two, he finds 

 all the fossil remains in the strata to be species then livuig in the adjoin- 

 ing sea, and he classes the whole series of beds as post-pliocene. But 

 the changes of conditions in the distant part of the ocean a, and the 

 constancy of conditions in the part b were synchronous ; and thus it 

 would appear that, taking the proportions of living species of shells 

 in the beds of the two parts as a standard, older and newer pliocene 

 deposits were forming in the part a, and post-pliocene in the part b at 

 one and the same time, in distant parts of the earth. If therefore 

 the above reasoning be correct, it seems to follow, that while pro- 

 portions of living species of shells constitute a sound principle for 

 discriminating changes of time, when accompanied by changes of 

 condition, over limited areas, two pliocene deposits at distant parts 

 of the earth's surface may not be certain evidence of synchronism. 

 In support of these views, I will quote the concluding sentence of the 

 fourth chapter of Mr. Darwin's new work, to which I shall after- 

 wards refer at some length, viz. — " The facts here given show how 

 cautious we ought to be in judging of the antiquity of a formation 

 from even a great amount of difference between the extinct and living 



