Chap. XIII. 
MORPHOLOGY. 
435 
natural selection of successive slight modifications^—• 
each modification being profitable in some way to the 
modified form, but often aifecting by correlation of 
growth other parts of the organisation. In changes 
of this nature, there will be little or no tendency to 
modify the original pattern, or to transpose parts. The 
bones of a limb might be shortened and widened to any 
extent, and become gradually enveloped in thick mem¬ 
brane, so as to serve as a fin; or a webbed foot might 
have all its bones, or certain bones, lengthened to any 
extent, and the membrane connecting them increased 
to any extent, so as to serve as a wing: yet in all this 
great amount of modification there will be no tendency 
to alter the framework of bones or the relative con¬ 
nexion of the several parts. If we suppose that the 
ancient progenitor, the archetype as it may be called, of 
all mammals, had its limbs constructed on the existing 
general pattern, for whatever purpose they served, we 
can at once perceive the plain signification of the homo¬ 
logous construction of the limbs throughout the whole 
class. So with the mouths of insects, we have only to 
suppose that their common progenitor had an upper 
lip, mandibles, and two pair of maxillae, these parts 
being perhaps very simple in form; and then natural 
selection, acting on some originally created form, will 
account for the infinite diversity in structure and func¬ 
tion of the mouths of insects. Nevertheless, it is con¬ 
ceivable that the general pattern of an organ might 
become so much obscured as to be finally lost, by the 
atrophy and ultimately by the complete abortion of cer¬ 
tain parts, by the soldering together of other parts, and 
by the doubling or multiplication of others,—variations 
which we know to be within the limits of possibility. 
In the paddles of the extinct gigantic sea-lizards, and 
in the mouths of certain suctorial crustaceans, the 
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