RECAPITULATION. 461 



perfect and liable to mistakes, ana 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 resem- 

 blance 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 similar- 

 ity would be a strange fact, if species had been independently 

 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 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 from the principle of natural 

 selection ; for old forms are supplanted by new and improved 

 forms. Neither single species nor groups of species reap- 

 pear 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 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 beings 

 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 

 more improved forms have conquered the older and less 

 improved forms in the struggle for life ; they have also 

 generally ha4 j&eir organs more specialized £9? 3iff§re»tf 



