110 AFFINITIES OF EXTINCT SPECIES. [Chap. XL 



the reader to turn to the diagram in the fourth chapter. 

 We may suppose that the numbered letters in italics 

 represent genera; and the dotted lines diverging from 

 them the species in each genus. The diagram is much 

 too simple, too few genera and too few species be- 

 ing given, but this is unimportant for us. The hori- 

 zontal lines may represent successive geological forma- 

 tions, and all the forms beneath the uppermost line may 

 be considered as extinct. The three existing genera a", 

 5", p", will form a small family; &^* and /" a closely 

 allied family or sub-family; and 0", e", m", 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 char- 

 acter 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^*- 



All the many forms, extinct and recent, descended 

 from (A), make, as before remarked, one order; and 



