RECAPITULATION. 491 



being not perfect and liable to mistakes, and at many 

 instincts causing other animals to suffer. 



If species be only well-marked and permanent varieties, 

 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 absorbed into each 

 other by successive crosses, and in other such points — as do 

 the crossed offspring of acknowledged varieties. This 

 similarity would be a strange fact, if species had been in- 

 dependently created and varieties had been produced 

 through secondary laws. 



If we admit that the geological record is imperfect to an 

 extreme degree, then the facts, which the record does give, 

 strongly support the theory of descent with modification. 

 New species have come on the stage slowly and at succes- 

 sive 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 from the prin- 

 ciple 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 dom- 

 inant forms, with the slow modification of their descend- 

 ants, causes the forms of life, after long intervals of time, 

 to appear as if they had changed simultaneously through- 

 out 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 beingg 

 can be classed with all recent beings, naturally follows from 

 the living and the extinct being the offspring of common 

 parents. As species have generally diverged in character 

 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 between existing groups. 

 Recent forms are generally looked upon as being, on the 

 whole, higher in the scale of organization than ancient 

 forms; and they must be higher, in so far as the later and 



