GEOLCXJICAL SUCCESSION OF ORGANIC BEINGS 311 



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 (fl^* to w^*), on the uppermost 

 line, be supposed to differ from each other by half-a-dozen im- 

 portant characters, then the families which existed at a 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 pro- 

 genitor. 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 

 represented in the diagram; for the groups will have been more 

 numerous; they will have endured for extremely unequal lengths 

 of time, and will have been modified in various degrees. As we 

 possess only the last volume of the geological record, and that in 

 a very broken condition, we have no right to expect, except in 

 rare cases, to fill up the wide intervals in the natural system, and 

 thus to 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 forma- 

 tions 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 palaeontologists is 

 frequently 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, are explained in a satisfac- 

 tory manner. And they are wholly inexplicable on any other view. 



On this same theory, it is evident that the fauna during any 

 one great period in the earth's history will be intermediate in 

 general character between that which preceded 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 be- 

 came still 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 



