120 THE ORIGIN OF SPECIES 



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 difier 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 paleontologists, is frequently the case. 



Thus, on the theory of descent with modification, 

 the main facts, with respect to the mutual afiinities 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 char- 

 acter 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 immigra- 

 tion of new forms from other regions, and for a large 

 amount of modification during the long and blank inter- 

 vals between the successive formations. Subject to these 

 allowances, the fauna of each geological period undoubt- 

 edly is intermediate in character between the preceding 

 and succeeding faunas. I need give only one instance, 

 namely, the manner in which the fossils of the Devonian 



