TIIK SUCfKSSION OF >TI:.\TII- 1 Kl K< " '.< 



peculiar genera would entitle a series of beds to the rank of a 

 >y.-ti-m ; but a general agreement will doubtless be reached in time. 

 Meanwhile the student will do well to regard our present claasiftaa- 

 tion as provisional, and to remember that, though divisional line-* 

 must be drawn somewhere, passage beds and transition groups are 

 of frequent occurrence in nature. 



2. Palceontological Evidence. But although we must rely mainly 

 on palfeontological evidence in separating one system from another, it 

 must not be supposed that every sudden change of fauna is to be 

 taken as introducing a new system, or that such a change always 

 indicates a period of non-deposition, and therefore a greater or lew 

 break in the succession ; unless this coincides with a physical break, 

 the mere fact of such a change does not necessarily prove any great 

 lapse of time. The change may be caused in several ways, either 

 by rapid depression, by a sudden immigration of new forms, or by 

 elevation with or without continuous deposition. 



It is conceivable, for instance, that rapid depression, causing a 

 considerable increase in the depth of the water, and removing the 

 area farther away from the coast line, would produce what has been 

 termed a palaeontological break in a comparatively short period of 

 time, and without any discontinuity of deposit. The beds below a 

 certain plane would embed the remains of a shore fauna, while 

 those above it would contain a deep-water assemblage, which might 

 be very different even if the deposits were of similar lithological 

 character. Depression, by the submergence of barriers, may also 

 cause much more extensive changes; we have only to con.-i'lcr 

 what would result from the submergence of certain rxi.-u-nt 

 isthmuses, such as those of Panama and Suez, to realise what must 

 have repeatedly happened in the past The balance of life in both 

 provinces would be altogether disturbed and upset, resulting in the 

 extinction of some forms, the inter-migration of others, and the 

 development of some new varieties and species. 



Moreover, an alteration in the physical geography of one area 

 may indirectly, but very greatly, affect the climate and life of 

 another distant area ; thus, if the whole isthmus of Panama were 

 now to be submerged, the Gulf Stream would probably be diverted 

 from its present course, the result of which would !* an immediate 

 refrigeration of the British climate and a consequent immigration 

 of Boreal and Arctic species into the Celtic province. Again, if 

 the Gulf Stream remained, but the opening into the A MIC ocean 

 were closed, the British climate would be ameliorated, Boreal 

 forms would disappear, and a more southern assemblage would 

 occupy the area. 



Elevation of the sea-bottom will produce similar direct changes 



