Chap. XIV. RECAPITULATION. 475 



yet should follow nearly the same instincts ; why the 

 thrush of South America, for instance, lines her nest 

 with mud like our British species. On the view of 

 instincts having been slowly acquired through natural 

 selection we need not marvel at some instincts being 

 apparently not perfect and liable to mistakes, and at 

 many instincts causing other animals to suffer. 



If species be only well-marked and permanent varie- 

 ties, we can at once see why their crossed offspring 

 should follow the same complex laws in their degrees 

 and kinds of resemblance to their parents, — in being ab- 

 sorbed into each other by successive crosses, and in other 

 such points, — as do the crossed offspring of acknow- 

 ledged varieties. On the other hand, these would be 

 strange facts if species have been independently created, 

 and varieties have been produced by secondary laws. 



If we admit that the geological record is imperfect 

 in an extreme degree, then such facts as the record 

 gives, support the theory of descent with modification. 

 New species have come on the stage slowly and at 

 successive intervals ; and the amount of change, after 

 equal intervals of time, is widely different in different 

 groups. The extinction of species and of whole groups 

 of species, which has played so conspicuous a part in the 

 history of the organic world, almost inevitably follows 

 on the principle of natural selection ; for old forms will 

 be supplanted by new and improved forms. Neither 

 single species nor groups of species reappear when the 

 chain of ordinary generation has once been broken. 

 The gradual diffusion of dominant forms, with the slow- 

 modification of their descendants, causes the forms of 

 life, after long intervals of time, to appear as if they 

 had changed simultaneously throughout the world. The 

 fact of the fossil remains of each formation being in 

 some degree intermediate in character between the 



