CHAP. VI GEOGRAPHICAL AND GEOLOGICAL CHANGES 103 



there are exceptions to the rule, we can generally trace 

 them to some changed conditions which have led to the 

 extinction of certain types. But when we go a little 

 further back, to the late or middle Tertiary deposits, we 

 almost always find, along with forms which might have 

 been the ancestors of some now living, others which are 

 only now found in remote regions and often in distinct 

 continents — clear indications of those extensive migrations 

 which have ever been going on. Every large island 

 contains in its animal inhabitants a record of the period 

 when it was last separated from the adjacent continent, 

 while some portions of existing continents still show by the 

 comparative poverty and speciality of their animals that 

 at no distant epoch they were cut off by arms of the sea 

 and formed islands. If the geological record were more 

 perfect, or even if we had as good a knowledge of that record 

 in all parts of the world as we have in Europe and North 

 America, we could arrive at much more accurate results 

 than we are able to do with our present very imperfect 

 knowledge of extinct forms of life ; but even with our 

 present scanty information we are able to throw much 

 light upon the past history of our globe and its inhabitants, 

 and can sketch out with confidence many of the changes 

 they must have undergone. 



Summary of Evidence for the General Permanence of 

 Conanents and Oceans. — As this question of the permanenc 

 of our continents or, rather, of the continental areas, lies at 

 the root of all our inquiries into the past changes of the 

 earth and its inhabitants, and as it is at present completely 

 ignored by many writers, and even by naturalists of 

 eminence, it will be well to summarise the various kinds of 

 evidence which go to establish it.^ We know as a fact 



^ In a review of Mr. T. Mellard Reade's Chemical Denudation and 

 Geological Time, in Nature (Oct. 2nd, 1879), the writer remarks as follows : — 

 ** One of tlie funny notions of some scientific thinkers meets with no favour 

 from Mr. Reade, whose geological knowledge is practical as well as theoretical. 

 They consider that because the older rocks contain nothing like the present 

 red clays, &c., of the ocean floor, that the oceans have always been in their 

 present positions. Mr. Reade points out that the first proposition is not 

 yet proved, and the distribution of animals and plants and the fact that 

 the bulk of the strata on land are of marine origin are opposed to the hypo- 



