Chap. X. TIIIIOUGIIOUT THE WOULD. ,^07 



of tlie species bclongiiijr to the same genera, yet the species 

 thetiisehes diircr in a manner very difTicult to account for, con- 

 sidcrin<x the proximit}- of tlic tuo areas, unless indeed it be 

 assumed that an isthmus separated two seas inhabited by dis- 

 tinct but contemporaneous faunas. Lyell has made similar 

 observations on some of the later tertiary formations. Barrande, 

 also, shows that there is a striking general parallelism in the 

 successive Silurian deposits of Bohemia and Scandinavia ; never- 

 theless he finds a surprising amount of dillcrence in the species. 

 If the several formations in these regions have not been de- 

 jiosited during the same exact periods — a formation in one re- 

 gion often corresponding with a blank interval in the other — 

 and if in both regions the species have gone on slowly chan- 

 ging during tlie accumulation of tlie several formations, and 

 (hning the long intervals of time between tliem ; in this case 

 the several formations in the two regions could be arranged in 

 the same order, in accordance with the general succession of 

 the forms of life, and the order would falsely appear to be strictly 

 parallel ; nevertheless the species would not be all the same in 

 the aiiparcntly corresponding stages in the two regions. 



On the Affinities of J^xtinct Species to each other, and to 

 Livinf/ J/ornis. 



Let us now look to the mutual affinities of extinct and liv- 

 ing species. They all fall into a few grand classes ; and this 

 fact is at once explain(Ml on the princij)le of descent. The more 

 ancient any form is, the more, as a general rule, it differs from 

 living forms. But, as Buckland long ago remarked, all extinct 

 species can be classed eitlier in still existing grou})s, or between 

 tliem. That the extinct forms of life help to fill up the inter- 

 vals between existing genera, families, and orders, cannot be 

 disputed. For if we confine our attention cither to the living 

 or to the extinct alone, the series is far less perfect than if we 

 combine both into one general system. With respect to the 

 vertebrata, whole pages could be filled with illustrations from 

 Owen, showing how extinct animals fall in between existing 

 groups. Cuvier ranked the Ruminants and Pachyderms, as the 

 two most distinct orders of mammals ; but Owen has discovered 

 so many fossil links, that he has had to alter th(; whole classifi- 

 cation, and has placed certain j)achyderms in the same sub-order 

 with nnuinants : for example, he dissolves by fine gradations 

 the apparently wide difference between the pig and the camel. 



