132 DR. J. STEPHENSON ON THE MORPHOLOGY, CLASSIFICATION, 



the changes have been opinions vary ; the tide sways backward 

 and forward, and no agreement has been reached. Zoologists 

 will remember that Wallace inclined at first to the opinion of 

 numerous and great changes — such fundamental changes as the 

 bridging of the Indian Ocean by the hypothetical continent 

 Lemuria ; but that later he came to believe in the essential 

 permanence of all the great ocean basins. On the whole, how- 

 ever, the zoologists are to be found among the bridge-builders, 

 and they have the company of some distinguished geologists ; 

 but it is perhaps true to say that geological opinion at present 

 is inclining to the theory of permanence. 



Needless to say, no one would deny vertical movements of the 

 order of 100 fathoms or so; no one could refuse to believe that 

 England had been united to the mainland, or that a large part of 

 the Malay Archipelago had been united to the continent of Asia. 

 And a rise of 100 fathoms would unite all the large masses of 

 land into one, with Australia as a doubtful exception ; seen in a 

 1ST. Polar projection, we should have a mass of land round the- 

 N. Pole, with three tongues, S. America, Africa, and Malaya 

 radiating outwards towards the S. Pole. The soundings are not 

 sufficient to determine whether there is a continuous bridge to 

 Australia above "bhe 100 fathom line or not. New Zealand, 

 Madagascar, the West Indies, and numerous small oceanic islands 

 would remain separate. A lowering of 100 fathoms would isolate 

 N. and S. America, Asia and Africa; and Europe woiild form a 

 complex of islands and peninsulas much like the East Indies 

 to-day. It is changes of this order that are- considered allowable 

 by the more conservative school, not such changes as would 

 bridge the N. or S. Atlantic or Pacific Oceans. 



What appears to have brought about something of a change of 

 opinion in recent years is the increasing support accorded to the 

 theory of isostasy. The earth's crust is in a condition of approxi- 

 mate equilibrium, the crust being less dense under the mountains 

 and continental masses in general, more dense under the ocean 

 floor — this is shown by the measurements of gravity. It may be- 

 too venturesome to say that the mountains float like icebergs in 

 water ; but the idea is that the land-masses project because they 

 are lighter, while the bed of the oceans has sunk because this 

 portion of the crust is heavier ; and without the most extensive 

 lateral motion of the matter of the crust the general arrangement 

 of continental masses and ocean cannot change. 



In addition to the measurements of gravity is the fact that 

 there are no abyssal deposits on the continental platforms 

 wherever these have been adequately studied : i. <?., the continents 

 have never been deeply submerged, though shallow seas from 

 time to time there may have been. And the continental shelf 

 is so marked, obvious, and universal a feature of the earth's 

 surface that it affords the strongest kind of evidence of the 

 antiquity of the ocean basins and the limits beyond which the 

 continents have not extended. I have mentioned the effect of 



