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and to fatten readily. The same varieties of the cabbage do not
yield abundant and nutritious foliage and a copious supply of oil
bearing seeds.”—(Origin of Species, p. ny.)
On the other hand, when one portion of a limb is
removed, the remaining portion is strengthened so as to do
the additional work thrown upon it.
“According to the interesting observations of M. Sedillot, when a
portion of one bone of the leg or fore-arm of an animal is removed,
and is not replaced by growth, the associated bone enlarges till it
attains a bulk equal to that of the two bones, of which it has to
perform the functions. This is best exhibited in dogs in which the
tibia has been removed ; the companion bone, which is naturally
almost filiform and not one-fifth the size of the other, soon acquires a
size equal to, or greater than, the tibia.”—(The Variation, vol. ii.,
p. 296.)
Thus correlated variation is the agent whereby one
structure of co-ordinated parts is transformed into another
structure with parts more or less otherwise co-ordinated.
But there is another cause which probably is quite as
efficient and far-reaching. I refer to the fact that the
co-ordinated parts of the structure are the result of a
process of development ; and in consequence of this fact
there is a close sympathy between certain associated
parts, the result being that when one part is modified,
other parts are unavoidably modified.
Those parts which are developed from corresponding
embryonic parts—as the arm, fore-leg, and wing of
different animqls, or the fore and hind limbs of the
same animal, parts which are at one period identical in
their structure, and necessarily exposed to the same
conditions—tend to vary in the same way. These parts
are called homologous, and the principle which expresses
the relation of one to the other is known as homology.
We have three kinds of homology—serial, represented by
the centipede ; lateral, represented by the two corres-