494 



RECAPITULA TION. 



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

 groups subordinate to groups, and with the extinct groups 

 often falli^^? in between the recent groups, is intelligible 

 on the th^ of natural selection with its contingencies of 

 extinction and divergence 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 charac- 

 ters derived from rudimentary parts, though of no service 

 to the beings, are often of high classificatory value; and 

 why embryological characters are often the most valuable 

 of all. The real affinities of all organic beings, in contra- 

 distinction to their adaptive resemblances, are due to inher- 

 itance or community of descent. The Natural System is a 

 genealogical arrangement, with the acquired grades of dif- 

 ference, marked by the terms, varieties, species, genera, 

 families, etc. ; 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 porjDoise, and leg of the horse — 

 the same number of vertebrae 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 simi- 

 larity 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 like- 

 wise, to a large extent, intelligible on the view of the grad- 

 ual modification 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 superven- 

 ing 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 should be so closely 

 similar, and so unlike the adult forms. We mav cease 

 marveling 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 branchiEe. 



