380 ORIGIN OF SPECIES 



ages ; and it would be difficult to prove the truth of the propo- 

 sition, for every now and then a living animal, as the Lepi- 

 dosiren, is discovered having affinities directed towards very 

 distinct groups. Yet if we compare the older Reptiles and 

 Batrachians, the older Fish, the older Cephalopods, and the 

 eocene Mammals, with the more recent members of the same 

 classes, we must admit that there is truth in the remark. 



Let us see how far these several facts and inferences ac- 

 cord with the theory of descent with modification. As the 

 subject is somewhat complex, I must request 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 being given, but this is unimportant for us. The 

 horizontal lines may represent successive geological forma- 

 tions, and all the forms beneath the uppermost line may be 

 considered as extinct. The three existing genera o" q^\ p^\ 

 will form a small family; t" and /" a closely allied family 

 or sub-family; and o'\ e'\ m" a third family. These three 

 families, together with the many extinct genera on the sev- 

 eral 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 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 pos- 

 sible, 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 this order. 



