We need not here consider how the bodies of some animals
first became divided into a series of segments, or how they became divided
into right and left sides, with corresponding organs, for such questions
are almost beyond investigation. It is, however, probable that some serial
structures are the result of cells multiplying by division, entailing the
multiplication of the parts developed from such cells. It must suffice for
our purpose to bear in mind that an indefinite repetition of the same part
or organ is the common characteristic, as Owen has remarked, of all low or
little specialised forms; therefore the unknown progenitor of the
Vertebrata probably possessed many vertebrae; the unknown progenitor of the
Articulata, many segments; and the unknown progenitor of flowering plants,
many leaves arranged in one or more spires. We have also formerly seen
that parts many times repeated are eminently liable to vary, not only in
number, but in form. Consequently such parts, being already present in
considerable numbers, and being highly variable, would naturally afford the
materials for adaptation to the most different purposes; yet they would
generally retain, through the force of inheritance, plain traces of their
original or fundamental resemblance. They would retain this resemblance
all the more, as the variations, which afforded the basis for their
subsequent modification through natural selection, would tend from the
first to be similar; the parts being at an early stage of growth alike, and
being subjected to nearly the same conditions.
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