DISTKIWTlnN IN I [ME. 25 



In any given locality, the succession of living forms may 



appear to be interrupted l>y numerous InvjiUs- tin associated 

 species in each fossiliterous bed Ix-in^ |uitc distinct 

 those above and those below them. Hut the tendency of all 

 palaeontologies] investigation is to show that these breaks are 

 only apparent, and arise from tin- incompleteness of the series 

 of remains which happens to have been preserved in any gi\ -n 

 locality. As the area over which accurate geological in\ 

 Cations have been carried on extends, and as i h < rous 



rocks found in one locality fill up the gaps left in another, so 

 do the abrupt dernarkatious between the faunae and flora} of 

 successive epochs disappear a certain proportion of the gen- 

 era and even of the species of every period, great or small, 

 being found to be continued for a longer or shorter time 

 the next succeeding period. It is evident, in fact, that the 

 changes in the living population of the globe which have taken 

 place during its history have been effected, not by the sud- 

 den replacement of one set of living beings by another, but 

 by a process of slow and gradual introduction of new species, 

 accompanied by the extinction of the older forms. 



It is a remarkable circumstance that, in all parts of the 

 globe in which fossiliferous rocks have yet been exami 

 the successive terms of the series of living forms which have 

 thus succeeded one another are analogous. The life of the 

 Mesozoic epoch is everywhere characterized by the abundance 

 of some groups of species of which no trace is to be found in 

 either earlier or later formations ; and the like is true of the 

 Palaeozoic epoch. Hence it follows, not only tint there has 

 been a succession of species, but that the general nature of 

 that succession has been the same all over the globe ; and it 

 is on this ground that fossils are so important to the geologist 

 as marks of the relative age of rocks. 



The determination of the morphological relations of the 

 species which have thus succeeded one another, is a problem 

 of profound importance and difficulty, the solution of which, 

 however, is already clearly indicated. For, in several cases, 

 it is possible to show that, in the same geographical area, a 

 form A, which existed during a certain geological epoch, 

 been replaced by another form B, at a later period; and that 

 this form B has been replaced, still later, by a third form C. 

 When these forms, A, B, and C, are compared together they 

 are found to be organized upon the same plan, and to be 

 very similar even in most of the details of their struct \ 

 but B differs from A by a slight modification of some ot 

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