112 AFFINITIES OF EXTINCT SPECIES. [Chap. XI 



each other by a less number of characters; 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 compli- 

 cated than is represented in the diagram; for the groups 

 will have been more numerous; they will have en- 

 dured for extreraely 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 for- 

 mations 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 palseontologists 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 inexpUeable on any other view. 



On this same theory, it is evident 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 spe- 



