358 



NA TURE 



[Feb. 17, 188 1 



Inevitable as was this belief in the early days of 

 geology, and lirmly as it still maintains its hold, it is 

 unquestionably based upon a partial view and erroneous 

 interpretation of the facts. This has for some years 

 been recognised by a few writers, and will before long be 

 generally acknowledged. Instead of shifting their places 

 on the earth's surface, continents, so far as the evidence 

 of their history can be gleaned, have been wonderfully 

 persistent. 



This conclusion is reached by many different paths 

 of inquiry. Of these it may suffice to notice here only 

 two. (i)The rocks of which the greater part of the 

 dry land consists, are upraised marine sediments. But 

 their materials were derived from the waste of neighbour- 

 ing dry land. They everywhere contain indications of 

 the proximity of that land, and even reveal terrestrial 

 surfaces, such as rippled-marked and rain-pitted shores, 

 in the very midst of marine formations. Nowhere do 

 they present indications of really deep water. (2) An 

 examination of the floor of the present ocean proves that 

 the sediment now removed from the surface of the 

 continent is deposited in the shallower waters within 

 150 or 200 miles from land. Beyond this limit ter- 

 restrial sediment ceases to be transported and depo- 

 sited, its place being taken by organic accumulations 

 and by peculiar red and grey " clays " in which the 

 inorganic material is mainly of volcanic origin, and must 

 gather on the bottom with almost inconceivable slowness. 

 This grouping of the detritus, derived from the degrada- 

 tion of the land, is evidently the only one possible, and it 

 has now been abundantly demonstrated by recent deep- 

 sea researches. We may be sure also that it must always 

 have obtained in every geological period. The coarser 

 and more lenticular sheets of sediment have accumulated 

 nearest to the sources of supply, that is to the shores of 

 the land ; while the finer and more wide-spread silts have 

 been spread over the farther and deeper tracts of that still 

 comparatively narrow belt of sea to which sedimentation 

 has always been mainly confined. To hasty readers it 

 will seem an obvious and ridiculous paradox to maintain 

 that the continents have been permanent throughout 

 geological time, and yet to admit that probably no part of 

 their surface has not been many times submerged beneath 

 the ocean. Further reflection, however, and better ac- 

 quaintance with the facts will convince every candid 

 inquirer that the paradox is only in appearance. The 

 continental ridges have been the great lines of terrestrial 

 movement from the dawn of geological history. They have 

 continually been undergoing disturbance; one portion has 

 been equably upraised, another has been convulsed and 

 corrugated, a third has been depressed. Every part of their 

 surface has been subject to these changes. Moreover every 

 portion of the crust which has risen above the sea- 

 level has been exposed to the unremitting attacks of 

 the subaerial agents of destruction. Again and again 

 the sohd bulk of the continents has been reduced to mere 

 detritus and has been spread over the sea-bottom. And 

 yet the continental ridges have never ceased to exist. 

 Their disappearance would necessarily have been fol- 

 lowed by the cessation of sedimentary accumulation. 

 The character of their component rocks however teaches 

 that, whether by the operation of underground movements 

 or by the action of superficial causes, the land has been 



continually wandering, as it were, to and fro across the 

 continental areas, disappearing beneath the sea in one 

 region, reappearing from the sea in another. In one 

 sense of course it may be said that land and sea have 

 been continually changing places. But the submerged 

 land has not become truly a part of the oceanic realm. 

 The waters covering it have been mere prolongations of 

 the upper layers of the ocean, like the Mediterranean, 

 Black, and Caspian Seas of the present day. An eleva- 

 tion or depression of a few hundred feet, sufficed to turn 

 wide tracts into land or into water. But such oscilla- 

 tions made no real change in the essential position of 

 the grand aboriginal oceanic basins and continental 

 ridges. 



Mr. Wallace has thoroughly grasped the truth and 

 significance of these averments, and has not been slow 

 to perceive their fundamental importance in the history 

 of terrestrial floras and faunas. He finds that they furnish 

 new and unexpected assistance to the student of biological 

 evolution, and indeed form a necessary part of the 

 doctrine. " It is impossible," he says, "to exaggerate or 

 even adequately to conceive the effect of these endless 

 [terrestrial] mutations on the animal world. Slowly but 

 surely the whole population of living things must have 

 been driven backward and forward from east to west or 

 from north to south, from one side of a continent or a 

 hemisphere to the other. Owing to the remarkable 

 continuity of all the land masses, animals and plants 

 must have often been compelled to migrate into other 

 continents, where in the struggle for existence under new 

 conditions many would succumb ; while such as were 

 able to survive would constitute those widespread groups 

 whose distribution often puzzles us. Owing to the repeated 

 isolation of portions of continents for long periods, special 

 forms of life would have time to be developed, which 

 when again brought into competition with the fauna 

 from which they had been separated, would cause fresh 

 struggles of ever-increasing complexity, and thus lead to 

 the development and preservation of every weapon, 

 every habit, and every instinct which could in any way 

 conduce to the safety and preservation of the several 

 species." 



Besides interchanges of sea and land Mr. Wallace lays 

 great stress upon former vicissitudes of climate as agents 

 in the modification of plant and animal life. He has 

 discussed this subject with great detail and offers an 

 original explanation of the causes of secular changes of 

 climate. Adopting generally Dr. CroU's views as to the 

 relation between the Glacial period and the excentricity 

 of the earth's orbit, he introduces into them certain 

 modifications and limitations. If, he argues, the effects 

 of a high excentricity have always been shown in great 

 Polar refrigeration and a general lowering of the tem- 

 perature in the hemisphere whose winter occurred in 

 aplicUon, there ought to be geological evidence of the 

 change. He confesses however that although indications 

 of local ice-action have been noticed in different geologi- 

 cal formations, even as far back as old PaUcozoic deposits, 

 there is certainly no trace of such general glaciations as 

 the theory would lead us to expect. Not only so, but 

 the testimony of organic remains is everywhere and un- 

 mistakably against the theory. He concludes, therefore, 

 that while the astronomical influences must unquestion- 



