RECAPITULATION 297 



after equal intervals of time, is widely different in dif- 

 ferent 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 inevi- 

 tably follows from the principle of natural selection; for 

 old forms are supplanted by new and improved forms. 

 Neither single species nor groups of species reappear 

 when the chain of ordinary generation is once 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 fos- 

 sils in the formations above and below, is simply ex- 

 plained by their intermediate position in the chain of 

 descent. The grand fact that all extinct beings can be 

 classed with a,!! recent beings, naturally follows from the 

 living and the extinct being the offspring of common 

 parents. As species have generally diverged in charac- 

 ter during their long course of descent and modification, 

 we can understand why it is that the more ancient forms, 

 or early progenitors of each group, so often occupy a 

 position in some degree intermediate betweeii existing 

 groups. Recent forms are generally looked upon as be- 

 ing, on the whole, higher in the scale of organization 

 than ancient forms; and they must be higher, in so far 

 as the later and more improved forms have conquered 

 the older and less improved forms in the struggle for 

 life; they have also generally had their organs more spe- 

 cialized for different functions. This fact is perfectly 

 compatible with numerous beings still retaining simple 



