AFFINITIES OF EXTINCT SPECIES. 359 



to turn to the diagram in the fourth chapter. "We may 

 suppose that the numbered letters in itahcs represent 

 genera, and the dotted lines diverging from them the spe- 

 cies in each genus. The diagram is much too simple, too 

 few genera and too few species being given, but this is un- 

 important for us. The horizontal lines may represent suc- 

 cessive geological formations, and all the forms beneath 

 the uppermost line may be considered as extinct. The 

 three existing genera «**, gi*, jt?**, will form a small 

 family; 51* and/^*, a closely allied family or subfamily, 

 and fli*, e^*, wji*, 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 progenitor. On the principle of the 

 continued tendency to divergence of character, which was 

 formerly illustrated 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 necessary contingency; it depends solely on the descend- 

 ants 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 thronghout a vast period the same gen- 

 eral characteristics. This is represented in the diagram by 

 thejletter 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 diverg- 

 ence 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, 



