118 THE ORIGIN OF SPECIES 



by this diagram, the more recent any form is, the more 

 it will generally 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 neces- 

 sary 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 dia- 

 gram by the letter f'*. 



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 diver- 

 gence of character, has become divided into several sub- 

 families and families, some of which are supposed to 

 have perished at different periods, and som« 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 instance, the genera a\ a*, a'", /% m% m*, 

 m% 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 certain 

 pachyderms. Yet he who objected to consider as inter- 



