The Origin of Species 



large extent, intelligible on the view of the 

 gradual modification of parts or organs, which 

 were aboriginally alike in an early progenitor in 

 each of these classes. On the principle of suc- 

 cessive variations not always supervening at an 

 early age, and being inherited at a corresponding 

 not early period of life, we clearly sec why the 

 embryos of mammals, birds, reptiles, and fishes 

 should be so closely similar, and so unlike the 

 adult forms. We may cease marvelling at the 

 embryo of an air-breathing mammal or bird 

 having branchial slits and arteries running in 

 loops, like those of a fish which has to breathe the 

 air dissolved in water by the aid of well-developed 

 branchiae [gills]. 



Disuse, aided sometimes by natural selection, 

 will often have reduced organs when rendered 

 useless under changed habits or conditions of 

 life; and we can understand on this view the 

 meaning of rudimentary organs. But disuse 

 and selection will generally act on each creature, 

 when it has come to maturity and has to play its 

 full part in the struggle for existence, and will 

 thus have little power in an organ during early 

 life; hence the organ will not be reduced or ren- 

 dered, rudimentary at this early age. The calf, 

 for instance, has inherited teeth, which never cut 

 through the gums of the upper jaw, from an early 

 progenitor having well-developed teeth; and we 

 may believe, that the teeth in the mature animal 

 were formerly reduced by disuse, (.wing to the 

 tongue and palate, or lips, having become ex- 

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