394 RECAPITULATION. [CHAP. XV. 



The fact, as we have seen, that all past and present organic 

 beings can be arranged within a few great classes, in groups 

 subordinate to groups, and with the extinct groups often falling 

 in between the recent groups, is intelligible on the theory of 

 natural selection with its contingencies of extinction and diver- 

 gence of character. On these same principles we see how it is, 

 that the mutual affinities of the forms within each class are so 

 complex and circuitous. We see why certain characters are far 

 more serviceable than others for classification; why adaptive 

 characters, though of paramount importance to the beings, are of 

 hardly any importance in classification; why characters derived 

 from rudimentary parts, though of no service to the beings, are 

 often of high classificatory value ; and why embryological charac- 

 ters are often the most valuable of all. The real affinities of 

 all organic beings, in contradistinction to their adaptive resem- 

 blances, are due to inheritance or community of descent. The 

 Natural System is a genealogical arrangement, with the acquired 

 grades of difference, marked by the terms, varieties, species, 

 genera, families, kc. ; and we have to discover the lines of descent 

 by the most permanent characters whatever they may be and of 

 however slight vital importance. 



The similar framework of bones in the hand of a man, wing of 

 a bat, fin of the porpoise, and leg of the horse, the same number 

 of vertebras forming the neck of the giraffe and of the elephant, 

 and innumerable 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 in the 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, 

 to a large extent, intelligible on the view of the gradual modifica- 

 tion of parts or organs, which were aboriginally alike in an early 

 progenitor in each of these classes. On the principle of successive 

 variations not always supervening at an early age, and being in- 

 herited at a corresponding not early period of life, we clearly see 

 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. 



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 mean- 

 ing 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 



