CHAP. XI.] AFFINITIES OF EXTINCT SPECIES. 285 



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 formations, and all the forms beneath the 

 uppermost line may be considered as extinct. The three existing 

 genera a 14 , q 14 , p u , will form a small family ; b u and f lt a closely 

 allied family or sub-family ; and o 14 , e 14 , m u , 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 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 differ most 

 from existing forms. We must not, however, assume that diver- 

 gence 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 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 M . 



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 divergence 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 forma- 

 tions, 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 1 , a 5 , a 10 , /*, m 3 , m e , m 9 , 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 intermediate the extinct 

 genera, which thus link together the living genera of three 

 families, would be partly justified, for they are intermediate, not 

 directly, but only by a long and circuitous course through many 

 widely different forms. If many extinct forms were to be dis- 

 covered above one of the middle horizontal lines or geological 

 formations for instance, above No. VI. but none from beneath 



