428 EECAPITDLATION, Chap. X[V. 



or coinimniity of descent. The Natural System is a genealo- 

 gical arrangement, with the acquired grades of difference, 

 marked by tlic terms, varieties, species, genera, families, etc. ; 

 and we kavc to discover the lines of descent by the most per- 

 manent characters, whatever they may be and of however slight 

 vital importance. 



The singular framework of bones in the hand of a man, 

 wing of a bat, fin of the porpoise, and leg of the horse — the same 

 ninuber 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 suc- 

 cessive modifications. Tlic similarity of pattern in the w'ing 

 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 intelligible on the view of the 

 gradual modification of parts or organs, which were aborigi- 

 nally 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 inherited at a corresponding not early 

 period of life, we clearly see why the embryos of mammals, 

 birds, reptiles, and fishes, are 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 air dissolved in water by the aid of well-developed 

 branchiiv. 



Disuse, aided sometimes by natural selection, has often 

 reduced organs when they have become useless under changed 

 halnts or conditions of life ; and we can clearly understand on 

 this vfew 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 ex- 

 istence, and Avill thus have little power on an organ during 

 early life ; hence the organ will not be reduced or rendered 

 rudimentary at this early age. The calf, for instance, has in- 

 herited teeth, which never cut through the gums of tlie upper 

 jaw, from an early progenitor having well-developed teeth ; 

 and we may believe, that the teeth in the mature animal were 

 reduced, during successive generations, by disuse, or by the 

 tongue and palate, or lips, ha^^ng become better fitted by 

 natural selection to browse without their aid ; whereas in the 

 calf, tlie teeth have been left imtouched by selection or disuse, 

 and on the principle of inheritance at corresponding ages have 



