GEOLOGICAL SUCCESSION OF ORGANIC BEINGS llt> 



mediate the extinct genera, which thus link together the 

 living genera of three families, would be partly justified, 

 for they are intermediate, not 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. YI. — but none from 

 beneath this line, then only two of the families (those on 

 the left hand, a'*, etc., and 6", 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'* to m'*), on 

 the uppermost line, be supposed to differ from each 

 other by half a dozen important characters, then the 

 families which existed at the period marked VI. would 

 certainly have differed from each other by a less number 

 of characters; for they would at this early stage of de- 

 scent 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 be- 

 tween 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 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 



