AFFINITIES OF EXTINCT SPECIES 381 



from the continued effects of extinction and divergence of 

 character, has become divided into several sub-families and 

 families, some of which are supposed to have perished at 

 different periods, and some to have endured to the present 

 day. 



By looking at the diagram we can see that if many of the 

 extinct forms supposed to be imbedded in the successive 

 formations, were discovered at several points low down in 

 the series, the three existing families on the uppermost line 

 would be rendered less distinct from each other. If, for in- 

 stance, the genera a\ d" , a", /", »^^ »»", 7n', were disinterred, 

 these three families would be so closely linked together that 

 they probably would have to be united into one great fam- 

 ily, in nearly the same manner as has occurred with rumi- 

 nants and certain pachyderms. Yet he who objected to con- 

 sider as intermediate 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 dift'ercnt 

 forms. If many extinct forms were to be discovered above 

 one of the horizontal lines or geological formations — for in- 

 stance, above No. VI. — but none from beneath this line, then 

 only two of the families (those on the left hand, a\ &c., and 

 fc," &c.) 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 

 *»"), on the uppermost line, be supposed to differ from each 

 other by half-a-dozen important characters, then the fami- 

 lies which existed at the period marked VI. would certainly 

 have differed from each other by a less number of char- 

 acters; 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 



