Chap. XIV. RECAPITULATION. 479 



importance in classification ; why characters derived 

 from rudimentary parts, though of no service to the 

 being, are often of high classificatory value ; and why 

 embryological characters are the most valuable of all. 

 The real affinities of all organic beings are due to inhe- 

 ritance or community of descent. The natural system 

 is a genealogical arrangement, in which we have to 

 discover the lines of descent by the most permanent 

 characters, however slight their vital importance 

 may be. 



The framework of bones being the same in the hand 

 of a man, wing of a bat, fin of the porpoise, and leg of 

 the horse, — the same number of vertebrae forming the 

 neck of the giraffe and of the elephant, — and innu- 

 merable other such facts, at once explain themselves on 

 the theory of descent with slow and slight successive 

 modifications. The similarity of pattern in the wing and 

 leg of a bat, though used for such different purpose, — in 

 the jaws and legs of a crab, — in the petals, stamens, and 

 pistils of a flower, is likewise intelligible on the view of 

 the gradual modification of parts or organs, which were 

 alike in the early progenitor of each class. On the 

 principle of successive variations not always supervening 

 at an early age, and being inherited at a corresponding 

 not early period of life, we can clearly see why the em- 

 bryos of mammals, birds, reptiles, and fishes should be 

 so closely alike, and should be 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 in a fish which has 

 to breathe the air dissolved in water, by the aid of well- 

 developed branchia). 



Disuse, aided sometimes by natural selection, will 

 often tend to reduce an organ, when it lias become 

 useless by changed habits or under changed conditions 



