332 GEOLOGICAL SUCCESSION. Chap. X. 



modified in relation to its slightly altered conditions of 

 life, and yet retain throughout a vast period the same 

 general characteristics. This is represented in the dia- 

 gram by the letter f u . 



All the many forms, extinct and recent, descended 

 from A, make, as before remarked, one order ; and this 

 order, from the continued effects of extinction and 

 divergence of character, has become divided into se- 

 veral 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 embedded 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 instance, the genera a 1 , a 5 , a 1 °,/ 8 , m 8 , m 6 , m 9 , 

 were disinterred, these three families would be so closely 

 linked together that they probably would have to be 

 united into one great family, in nearly the same manner 

 as has occurred with ruminants and pachyderms. Yet 

 he who objected to call the extinct genera, which thus 

 linked the living genera of three families together, inter- 

 mediate in character, would be justified, as they are 

 intermediate, not directly, but only by a long and cir- 

 cuitous 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 the two families on the left hand 

 (namely, a u , &c, and b u , &c.) would have to be united 

 into one family; and the two other families (namely, 

 a u to/ 14 now including five genera, and o 14 to m u ) Mould 

 yet remain distinct. These two families, however, would 

 be less distinct from each other than they were before the 



