428 ON THE ORIGIN OF SPECIES 



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 

 fossils in the formations above and below, is simply 

 explained by their intermediate position in the chain 

 of descent. The grand fact that all extinct organic 

 beings belong to the same system with recent beings, 

 falling either into the same or into intermediate groups, 

 follows from the living and the extinct being the 

 offspring of common parents. As the groups which 

 have descended from an ancient progenitor have gener- 

 ally diverged in character, the progenitor with its early 

 descendants will often be intermediate in character in 

 comparison with its later descendants ; and thus we 

 can see why the more ancient a fossil is, the oftener it 

 stands in some degree intermediate between existing 

 and allied groups. Recent forms are generally looked 

 at as being, in some vague sense, higher than ancient 

 and extinct forms ; and they are in so far higher as 

 the later and more improved forms have conquered the 

 older and less improved organic beings in the struggle 

 for life. Lastly, the law of the long endurance of 

 allied forms on the same continent, — of marsupials in 

 Australia, of edentata in America, and other such 

 cases, — is intelligible, for within a confined country, 

 the recent and the extinct will naturally be allied 

 by descent 



Looking to geographical distribution, if we admit 

 that there has been during the long course of ages 

 much migration from one part of the world to another, 

 owing to former climatal and geographical changes 

 and to the many occasional and unknown means of 

 dispersal, then we can understand, on the theory of 



