382 ORIGIN OF SPECIES 



unequal lengths of time, and will have been modified in vari- 

 ous degrees. As we possess only the last volume of the geo- 

 logical 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 peri- 

 ods, undergone much modification, should in the older for- 

 mations 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 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 inter- 

 mediate 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 modi- 

 fied 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 

 interval between the successive formations. Subject to these 

 allowances, the fauna of each geological period undoubtedly 

 is intermediate in character, between the preceding and suc- 

 ceeding 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 those 

 of the overlying carboniferous, and underlying Silurian sys- 

 tems. But each fauna is not necessarily exactly intermediate, 



