310 AFFINITIES OF EXTINX'T SrFX'IKS. Chap. X. 



be rendered less distinct from each otlior. If, for instance, the 

 g'enera «', «V ^'"5 /*» ''^*» ^''^S ''^'^ Averc disinterred, these three 

 families would be so closely linked together that they proba- 

 bly M'ould have to be xniited into one great family, in nearly 

 the same manner as has occurred -with ruminants and certain 

 pach^'dtM-ms. Yet he who objected to call the extinct genera 

 which thus linked the living genera of three families together, 

 intermediate in character, would be justified, as they are inter- 

 mediate, not directly, but only by a long and circuitous course 

 through many "vndely-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 beneatli this line, then only two of the families 

 (those on the left hand, «'*, etc., and b^*, etc.) would have to be 

 imited into one ; and there wovild 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 («" to ni^*), on the uppermost line, 

 be supposed to differ from each other by half a dozen impor- 

 tant characters, then the families which existed at the peiiod 

 marked VI. would certainly have differed from each other by 

 a less numljer 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 some slight degree intermediate in character be- 

 tween their modilled descendants, or between their collateral 

 relations. 



In Nature the case will be far more complicates! 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 Avill have been modified in various degrees. 

 As we possess only the last volume of the geological record, 

 and that in a very broken condition, Ave have no right to expect, 

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

 system, and thus imife distinct fiimilies or orders. All that we 

 have a right to expect is, that those groups which have within 

 knoA\ni 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 paleontologists, is frequently the case. 



Thus, on the thcorv of dcsrcnt witli modification, the main 



