PRE-DETERMINATION IN NATURE 33! 



have to deal with here, but permanence of position along with 

 change of elevation ; and this modified by the fact that there 

 have always been mountain ridges, internal plateaus, and mar- 

 ginal areas affected in various ways by the vertical movement 

 of the land. Further, the elevation and subsidence of the land 

 have not always been uniform, but often differential, while every 

 movement has tended to produce modifications of ocean cur- 

 rents and of atmospheric conditions. The whole subject, more 

 especially in its relations to life, thus becomes very complicated, 

 and it is perhaps in consequence of partial and imperfect views 

 on these points that so much diversity of opinion has arisen. 

 For example, it is evident that we can gain nothing by adding 

 to the continents those submerged margins delineated by 

 Murray in the Challenger reports, and which have in periods of 

 continental elevation themselves formed portions of the land. 

 Nor do we establish a case in favour of perished oceanic conti- 

 nents by the argument that they are needed to furnish the 

 materials of marginal mountains which are due to the con- 

 tinuous sweeping of arctic material to the south by currents, as 

 we see in the coast of North America to-day. Nor do we in- 

 validate the permanence of the continents by the bridges of 

 land, islands, and shallow water at various times thrown across 

 the Atlantic. The distribution of Cambrian Trilobites, as illus- 

 trated by Matthew, 1 seems to show a bridge of this kind in the 

 north in very early times, and similar evidence is furnished by 

 the animals and plants of the Devonian and Carboniferous, 

 and by the sea animals and plants of the later Tertiary and 

 modern. Gardener has postulated a southern bridge in the 

 region of the West Indies for the migrations of plants, and 

 Gregory has adduced the evidence of those conservative and 

 slow-moving creatures, the sea urchins, in favour of similar con- 

 nection in the West Indian region at two distinct periods of 

 time (the Lower Cretaceous and the Miocene Tertiary). But 

 1 Transactions Royal Society of Canada, 1892. 



