Chap. X. AFFINITIES OF EXTINCT SPECIES. 333 



discovery of the fossils. If, for instance, we suppose the 

 existing genera of the two families to differ from each 

 other bj a dozen characters, in this case the genera, at 

 the early period marked VI., would differ by a lesser 

 number of characters ; for at this early stage of descent 

 they have not diverged in character from the common 

 progenitor of the order, nearly so much as they sub- 

 sequently diverged. Thus it comes that ancient and 

 extinct genera are often in some slight degree inter- 

 mediate in character between their modified descendants, 

 or between their collateral relations. 



In nature the case will be far more complicated than 

 is represented in the diagram ; for the groups will have 

 been more numerous, they will have endured for ex- 

 tremely unequal lengths of time, and will have been 

 modified in various degrees. As we possess only the last 

 volume of the geological record, and that in a very broken 

 condition, we have no right to expect, except in very 

 rare cases, to fill up wide intervals in the natural system, 

 and thus unite distinct families or orders. All that 

 we have a right to expect, is that those groups, which 

 have within known geological periods undergone much 

 modification, should in the older formations make some 

 slight approach to each other ; so that the older mem- 

 bers should differ less from each other in some of their 

 characters than do the existing members of the same 

 groups ; and this by the concurrent evidence of our best 

 palaeontologists seems frequently to be the case. 



Thus, on the theory of descent with modification, the 

 main facts with respect to the mutual affinities of the 

 extinct forms of life to each other and to living forms, 

 seem to me explained in a satisfactory manner. And 

 they are wholly inexplicable on any other view. 



On this same theory, it is evident that the fauna of 

 any great period in the earth's history will be inter- 



