Chap. X. AFFINITIES OF EXTINCT SPECIES. 309 



Mammals, witli llio more recent members of the same classes, 

 we must admit that there is truth in tlie i-emark. 



Let us see how far these several facts and inferences accord 

 with tlic theory of descent with modification. As the subject 

 is somewhat complex, I must request the reader to turn to the 

 diafi^ram in the fourth chapter. We may suppose that the 

 numbered letters represent genera, and the dotted lines di- 

 verging from them the species in each genus. The diagram 

 is much too simple, too few genera and too few species being 

 given, but this is unimportant for us. The horizontal lines 

 may represent successive geological formations, and all the 

 forms beneath the uppermost line may be considered as ex- 

 tinct. The three existing genera, «'*, Q^*,])'*, will form a small 

 family ; i'* and _/"'*, a closely-allied family or sub-family ; and 

 o'*, e'*, Wi", a third lamily. These three families, together vvitli 

 the many extinct genera on the several lines of descent di- 

 verging irom the parent-form (A), will form an order; for all 

 Avill have inherited something in common from their ancient 

 and common ])rogenitor. On the principle of the continued 

 tendency to divergence of character, which was formerly illus- 

 trated 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 ditfer most 

 from existing forms. We must not, hoAvever, assume that di- 

 vergence 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 Natme. 

 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 let- 

 ter f'\ 



All the many forms, extinct and recent, descended from 

 (A), mak(% as Ix-fore remarked, one order;' and this order, from 

 the continued eflects of extinction and divergence of character, 

 has l)ecoine divided into several sul>families and families, some 

 of which are supposed to have perished at dilVercnt periods, 

 and some to have endured to the present da}'. 



By looking at the diagram we can sec that if many of the 

 extinct forms, supposed to be embedded in the successive for- 

 mations, were discovered at several jioints low down in the 

 series, the three existing families on the uppermost line would 



