Chap. XL] AFFINITIES OF EXTINCT SPECIES. Ill 



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 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^", 

 /^, m^, w^ m', 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 rumi- 

 nants 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 ex- 

 tinct 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^*, »&:c., and 6^*, &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^* to m^*), on the 

 uppermost line, be supposed to differ from each other 

 by half-a-dozen important characters, then the families 

 wliich existed at the period marked VI. would certainly 



