310 THE ORIGIN OF SPECIES 



uppermost line may be considered as extinct. The three existing 

 genera a^'*, ^^*, p^"^, will form a small family; b'^* and /^^, a closely 

 allied family or sub-family; and o^*, e^*, w^^, 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 com- 

 mon from their ancient progenitor. On the principle of the con- 

 tinued 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 de- 

 scendants 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 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 ex- 

 tinct forms supposed to be embedded 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, for instance, the genera a^, a^, a^^, 

 P, m^, m^, m^j 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 discovered above one of 

 the middle horizontal lines or geological formations — for instance, 

 above No. VI. — but none from beneath this line, then only two 

 of the families (those on the left hand, a}^, etc., and b^^, etc.) 



