RECAPITULATION AND CONCLUSION 431 



no service to the being 1 , 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 inheritance 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 im- 

 portance 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 vertebra? forming the 

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

 able 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 succes- 

 sive 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 embryos 

 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 branchiae. 



Disuse, aided sometimes by natural selection, will 

 often tend to reduce an organ, when it has become 

 useless by changed habits or under changed conditions 

 of life ; and we can clearly understand on this view the 

 meaning of rudimentary organs. But disuse and selec- 

 tion 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 

 of acting on an organ during early life ; hence the 



