Gunther, they are
probably remnants, consisting of the persistent axis of a fin, with the
lateral rays or branches aborted. The mammary glands of the
Ornithorhynchus may be considered, in comparison with the udders of a cow,
as in a nascent condition. The ovigerous frena of certain cirripedes,
which have ceased to give attachment to the ova and are feebly developed,
are nascent branchiae.
Rudimentary organs in the individuals of the same species are very liable
to vary in the degree of their development and in other respects. In
closely allied species, also, the extent to which the same organ has been
reduced occasionally differs much. This latter fact is well exemplified in
the state of the wings of female moths belonging to the same family.
Rudimentary organs may be utterly aborted; and this implies, that in
certain animals or plants, parts are entirely absent which analogy would
lead us to expect to find in them, and which are occasionally found in
monstrous individuals. Thus in most of the Scrophulariaceae the fifth
stamen is utterly aborted; yet we may conclude that a fifth stamen once
existed, for a rudiment of it is found in many species of the family, and
this rudiment occasionally becomes perfectly developed, as may sometimes be
seen in the common snap-dragon. In tracing the homologies of any part in
different members of the same class, nothing is more common, or, in order
fully to understand the relations of the parts, more useful than the
discovery of rudiments.
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