112 AFFINITIES OF EXTINCT SPECIES. [Chap. XI. 



have differed from each other by a less nurober of cha- 

 racters ; for they would at this early stage of descent 

 have diverged in a less degree from their common 

 progenitor. Thus it comes that ancient and extinct 

 genera are often in a greater or less degree intermediate 

 in character between their modified descendants, or 

 between their collateral relations. 



Under nature the process will be far more complicated 

 than is represented in the diagram ; for the groups will 

 have been more numerous ; they will have endured for 

 extremely 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 

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

 system, and thus to 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 

 members 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 is frequently 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, 

 are explained in a satisfactory manner. And they are 

 wholly inexplicable on any other \'iew. 



On this same theory, it is e\ident that the fauna 

 during any one great period in the earth's history will be 

 intermediate in general character between that which 

 preceded and that which succeeded it. Thus the species 



