It is, however, often difficult to distinguish between rudimentary and nascent organs; for we can judge only by analogy whether a part is capable of further development, in which case alone it deserves to be called nascent.
Organs in this condition will always be somewhat rare; for beings thus provided will commonly have been supplanted by their successors with the same organ in a more perfect state, and consequently will have become long ago extinct.

The wing of the penguin is of high service, acting as a fin; it may, therefore, represent the nascent state of the wing: not that I believe this to be the case; it is more probably a reduced organ, modified for a new function: the wing of the Apteryx, on the other hand, is quite useless, and is truly rudimentary.

Owen considers the simple filamentary limbs of the Lepidosiren as the "beginnings of organs which attain full functional development in higher vertebrates;" but, according to the view lately advocated by Dr.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.