38 INTRODUCTION. 



account in considering the value of fossils as tests of the age of 

 strata. Within a given area of such dimensions that we may sup- 

 pose it to have formed a single life-province, we shall undoubtedly 

 find that there is a recognisable succession of life-forms, so that 

 particular groups of rocks may be safely assigned, on the strength 

 of their contained fossils, to fixed places in the geological series, and 

 a definite chronological succession of the strata may thus be estab- 

 lished for the region examined. When we come, however, to com- 

 pare together the successive life-forms of widely remote areas, which 

 must be supposed to have always belonged to different life-provinces, 

 we cannot expect to find anything like a p7-ecise identity. We 

 shall probably be able to establish a general correspondence or 

 analogy, sufficient to establish a general parallelism of the successive 

 groups of strata in the two regions compared ; but it can only be in 

 exceptional circumstances that the fauna of a particular series of beds 

 in one region can possibly be largely identical with that of a coeval 

 series in a widely distant region. This principle is sufficiently estab- 

 lished by the simple consideration that the assemblages of animals 

 now existing simultaneously in different regions are so unlike each 

 other that we can by their means divide the earth's surface into a 

 number of definitely bounded " zoological provinces," and that there 

 is every reason to suppose that similar life-provinces have existed in 

 all the great geological periods of which the palaeontological history 

 has been preserved. If, on the other hand, we were to find that 

 the rocks deposited in any particular period of the earth's history con- 

 tained absolutely identical fossils in all parts of the world, we should 

 be forced to conclude that during that period there were no " zoologi- 

 cal provinces " developed, but that the entire terrestrial surface con- 

 stituted a single vast life-province inhabited by the same kinds of 

 animals and plants. Nothing that we have of actual evidence, de- 

 rived either from the past or the present, would, however, support 

 such a supposition ; but this point will be more clearly brought out 

 in dealing with the " contemporaneity " of strata in different regions. 



General Chronological Succession of the 

 Stratified Rocks. 



As the result of observations made upon the superposition of 

 rocks in different regions, from their mineral characters, and from 

 their included fossils, geologists have been able to divide the series 

 of the stratified rocks into a number of different divisions or " rock- 

 systems," each characterised, in any given region, by a genera/ 

 uniformity of mineral composition and by a special and peculiar 

 assemblage of life-forms, and each representing a " period " in the 

 earth's history. In every country in the world that has been geo- 



