DISTRIBUTION IN TIME. 25 



In any given locality, the succession of living forms may 

 appear to be interrupted by numerous breaks — the associated 

 species in each fossiliferous bed being quite distinct from 

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

 palaeontological investigation is to show that these breaks are 

 only apparent, and arise from the incompleteness of the series 

 of remains which happens to have been preserved in any given 

 locality. As the area over which accurate geological investi- 

 p-ations have been carried on extends, and as the fossiliferous 

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

 do the abrupt demarkations between the faunae and florae 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 into 

 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 examined, 

 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 that 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, has 

 been replaced by another form B, at a later period; and that 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

 verv similar even in most of the details of their structure; 

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



