

The Origin of Species 



thrushes of temperate and tropical South Amer- 

 ica, for instance, line their nests with mud like 

 our British species. On the view of instincts 

 having been slowly acquired through natural 

 selection, wc need not marvel at some instincts 

 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 see at once 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 independently created 

 and varieties had been produced through second- 

 ary laws. 



If we admit that the geological record is im- 

 perfect 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 conspicu- 

 ous 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 reappear when the chain of 

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