Chap. XL] AFFINITIES OF EXTINCT SPECIES. 305 



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

 numerous ; tbey 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 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 

 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 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 between 

 the successive formations. Subject to these allowances, the fauna 

 of each geological period undoubtedly is intermediate in character, 

 between the preceding and succeeding faunas. I need give only 

 one instance, namely, the manner in which the fossils of tho 

 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 systems. But each fauna is not necessarily exactly inter- 

 mediate, as unequal intervals of time have elapsed between con- 

 secutive formations. 



It is no real objection to the truth of the statement that the 

 fauna of each period as a whole is nearly intermediate in character 

 between the preceding and succeeding faunas, that certain genera 



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