338 AFFINITIES OF EXTINCT SPECIES. 



directly, but only by a long and circuitous course through 

 many widely different forms. If many extinct forms were 

 to be discovered above one of the middle horizontal lines 

 or geological formations — for instance, above No. VI. — 

 but none from beneath this line, then only two of the fami- 

 lies (those on the left hand, a 14 , etc., and b u , etc.) would 

 have to be united into one ; and there would remain two 

 families, which would be less distinct from each other than 

 they were before the discovery of the fossils. So, again, if 

 the three families formed of eight genera (a 14 to m u ), on 

 the uppermost line, be supposed to differ from each other 

 by half-a-dozen important characters, then the families 

 which existed at a period marked VI. would certainly have 

 differed from 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 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 con- 

 current 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 ex- 

 tinct forms of life to each other and to living forms, are 

 explained in a satisfactory manner. And they are wholly 

 inexplicable 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 interme- 

 diate in general character between that which preceded and 



