AFFINITIES OF EXTINCT SPECIES. 337 



portant for us. The horizontal lines may represent succes- 

 sive geological formations, and all the forms beneath the 

 uppermost line may be considered as extinct. The three 

 existing genera a u , q u , p 14 , will form a small family ; b li and 

 f u , a closely allied family or sub-family ; and o u , e u , m u , a 

 third family. These three families, together with the many 

 extinct genera on the several lines of descent diverging from 

 the parent form (A), will form an order, for all will have 

 inherited something in common from their ancient progen- 

 itor. On the principle of the continued tendency to diver- 

 gence of character, which was formerly illustrated by this 

 diagram, the more recent any form is, the more it will gener- 

 ally differ from its ancient progenitor. Hence, we can 

 understand the rule that the most ancient fossils differ most 

 from existing forms. We must not, however, assume that 

 divergence of character is a necessary contingency ; it 

 depends solely on the descendants from a species being 

 thus enabled to seize on many and different places in the 

 economy of nature. Therefore it is quite possible, as we 

 have seen in the case of some Silurian forms, that a species 

 might go on being slightly 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 diagram by the letter f 14 . 



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 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 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 10 , f 8 , ra 3 , m 6 , m*, were disin- 

 terred, 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 aud certain pachyderms. Yet he who 

 objected to consider as intermediate the extinct genera, 

 which thus link together the living genera of three fami- 

 lies, would be partly justified, for they are intermediate, nof( 



