286 AFFINITIES OF EXTINCT SPECIES. [CHAP. XL 



this line, then only two of the families (those on the left hand, 

 14 , &c., and b l \ &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 14 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 cha- 

 racter 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 geolo- 

 gical 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 palaeon- 

 tologists 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 

 satisfactory 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 became 

 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 



