GEOLOGICAL SUCCESSION 299 



last volume of the geological record, and that in a very 

 broken condition, we have no right to expect, except 

 in very rare cases, to fill up wide intervals in the 

 natural system, and thus unite distinct families or 

 orders. All that we have a right to expect, is that 

 those groups, which have within known geological 

 periods undergone much modification, should in the 

 older formations make some slight approach to each 

 other ; so that the older members should differ less 

 from each other in some of their characters than do 

 the existing members of the same groups ; and this by 

 the concurrent evidence of our best palaeontologi 

 seems frequently to be the case. 



Thus, on the theory of descent with modification, the 

 main facts with respect to the mutual affinities of the 

 extinct forms of life to each other and to living forms, 

 seem to me explained in a satisfactory manner. And 

 they are wholly inexplicable on any other view. 



On this same theory, it is evident that the fauna of 

 any great period in the earth's history will be inter- 

 mediate in general character between that which pre- 

 ceded and that which succeeded it. Thus, the species 

 which lived at the sixth great stage of descent in the 

 diagram are the modified offspring of those which lived 

 at the fifth stage, and are the parents of those which 

 became stiil more modified at the seventh stage ; hence 

 they could hardly fail to be nearly intermediate in 

 character between the forms of life above and below. 

 We must, however, allow for the entire extinction of 

 some preceding forms, and in any one region for the 

 immigration of new forms from other regions, and for 

 a large amount of modification, during the long and 

 blank intervals between the successive formations.. 

 Subject to these allowances, the fauna of each geo- 

 logical period undoubtedly is intermediate in char- 

 acter, between the preceding and succeeding faunas. 

 I need give only one instance, namely, the manner in 

 which the fossils of the Devonian system, when this 

 system was first discovered, were at once recognised by 

 palaeontologists as intermediate in character between 



