486 THE EVOLUTION OF LIFE. 



either way. Were the geological record complete, or did it, 

 as both Uniformitarians and Progressionists have commonly 

 assumed, give us traces of the earliest organic forms; the 

 evidence hence derived, for or against, would have had more 

 weight than any other evidence. As it is, all we can do is to 

 see whether such fragmentary evidence as remains, is con- 

 gruous with the hypothesis. 



Palasontology has shown that there is a " general relation 

 between lapse of time and divergence of organic forms " 

 (§ 107) ; and that " this divergence is comparatively slow and 

 continuous where there is continuity in the geological forma- 

 tions, but is sudden and comparatively wide wherever there 

 occurs a great break in the succession of strata. '^ 'Now this 

 is obviously what we should expect. The hypothesis implies 

 structural changes that are not sudden but gradual. Hence, 

 where conformable strata indicate a continuous record, we 

 may anticipate successions of forms only slightly different 

 from one another; while we may rationally look for marked 

 contrasts between the groups of forms fossilized in adjacent 

 strata, where there is evidence of a great blank in the record. 



The permanent disappearances of species, of genera, and 

 of orders, which we saw to be a fact tolerably-well established, 

 is also a fact for which the belief in evolution prepares us. 

 If later organic forms have in all cases descended from 

 earlier organic forms, and have diverged during their descent, 

 both from their prototypes and from one another; then it 

 follows that such of them as become extinct at any epoch, 

 will never re-appear at a subsequent epoch; since there can 

 never again arise a concurrence and succession of conditions 

 such as those under which each type was evolved. 



Though comparisons of ancient and modern organic forms, 

 prove that many types have persisted through enormous 

 periods of time, without undergoing great changes; it was 

 shown that such comparisons do not disprove the occurrence 

 in other organic forms, of changes great enough to produce 

 what are called different types. The result of inductive in- 



