304 AFFINITIES OF EXTINCT SPECIES. [Chap. XI 



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 u . 



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 cf the extinct 

 forms supposed to be imbedded 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 1 , a 5 , a 10 , 

 f s , m 3 , m 6 , 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 cir- 

 cuitous 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 14 , &c, and h u , &c.) would 

 have to be united into one ; and there would remain two families, 

 which would be less distinct from each other than they were 

 before the discovery of the fossils. So again if the three families 

 formed of eight genera (a w to m 14 ), on the uppermost line, be 

 supposed to differ from each other by half-a-dozen important 

 characters, then the families which existed at the period marked 

 VI. would certainly have differed from each other by a less number 

 of characters ; for they would at this early stage of descent have 

 diverged in a less degree from their common progenitor. Thus it 

 comes that ancient and extinct genera are often in a greater or less 

 degree intermediate in character between their modified descendants, 

 or between their collateral relations. 



Under nature the process will be far more complicated than is 



