670 Transactions. — Geology. 



it otherwise they would die. It is therefore a manifest 

 inipossibihty for the chmates and environment of the past 

 to have been similar to what they now are. We know 

 what the organic results are of our present environment, 

 such as have been brought about by differentiations and adap- 

 tations operating through all past time ; but, because these 

 are known, can it be asserted or even suggested that 

 similar causes operated to bring about the organic results 

 such as are found among the ruins and rearrangements of the 

 past ? Were the physical conditions of New Zealand to-day 

 similar to what they were when the Dinornis and Harpagornis 

 were found living, we might wonder what has caused their 

 disappearance at a comparatively recent period ; but the ope- 

 ration of the law which brought about the extinction of 

 Palseotherium in the Lower Tertiary, of Deinotherium in the 

 Pliocene, and of the Mastodon in the American drift, no doubt 

 brought about tire disaiDpearance of the noble avifauna of 

 New Zealand. The physical conditions in the earth are 

 widely unlike what they once were, and every remove, every 

 change, is necessarily accompanied by modifications and 

 adaptations in the organic world. Nor could anything else be 

 expected if we suppose that organic life always exists under 

 conditions best adapted to it. Every remove onward in time 

 is like the effect produced upon a people by great social or 

 political changes. To many, a social or political change 

 means succour, to others disaster ; but the impulse is never- 

 theless forward, as evolution always is. Viewed from this 

 standpoint, it is evident that life in each age, epoch, period, 

 and era was the best of its kind suited thereto ; but can it be 

 imagined that an estimate of earth-change such as is now in 

 operation can be taken as a basis in estimating changes in the 

 ages gone by — a complex differentiation by which to interpret 

 a simple one in the order of nature ? All pala^ontological evi- 

 dence implies simpler physical conditions in times gone by, 

 and a growing complexity as we approach Post-Tertiary times; 

 and it is therefore impossible, without running serious risk of 

 error, to base the physical changes of the past upon what we 

 now see in progress. As a geological factor denudation is of 

 high importance in estimating change, and it must have been 

 of yet liigher importance in the earlier periods of the earth. 

 The work done by moving water can only be imagined in part 

 by bringing into prominence the fact that not less than thirty 

 miles of sedimentary rocks are said to have been deposited 

 from the beginning of the Azoic era to the close of the Ter- 

 tiary. But from whence came so much material ? 



The average height of the land-surface of the earth at the 

 present time is not more than 2,500ft., or less than half a mile, 

 so that the thickness of the sedimentary rocks, as estimated 



