Chap. XIV. 
RECAPITULATION. 
479 
importance in classification; why characters derived 
from rudimentary parts, though of no service to the 
being, are often of high classificafcory value; and why 
embryological characters are the most valuable of all. 
The real affinities of all organic beings are due to in¬ 
heritance or community of descent. The natural system 
is a genealogical arrangement, in which we have to 
discover the lines of descent by the most perma¬ 
nent 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 prin¬ 
ciple 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 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 
