398 THE EVOLUTION of life. 



weight tlian 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. 



Palaeontology 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 expect to find successions of forms only slightly different 

 from one another ; while we may rationally look for consider- 

 able 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 

 obviously 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 particular 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 occur- 

 rence in organic forms, of changes great enough to produce 

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

 quiry we saw to be, that while a few modern higher types 

 yield signs of having been developed from ancient lower 

 types ; and while there are many modern types which may 



