Notes on the Development, Structure, and Origin of the Median and Paired Fins of Fish

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Abstract
A great deal has been written in the last few years about the structure, development, and origin of the paired fins of fish, yet two rival and incompatible theories are still prevalent. According to the theory put forth by Gegenbaur (14, 15,16), the paired fins have been derived from gill structures, the gill-arch having been modified into the limb-girdle, and the fin itself, with its skeleton, having been derived from the gill-flap or septum, and its supporting gill-rays. This may shortly be called the “gill-arch theory.” The second theory, that of Balfour (1, 2), Thacher (35), and Mivart (23), holds that the paired fins are of the same nature as the unpaired median fins. According to this view, the limbs have been derived from paired longitudinal fin-folds, in which skeletal supports, the radials, or somactidia (Lankester), became developed as in the median fins, and subsequently gave rise to the limb-girdles. This may be called the “lateral fin-fold theory.” Each of these theories may claim to have among its numerous supporters the names of some of the most eminent exponents of the morphology of the vertebrates. Dohrn (10), Haswell (20), Rabi (31), Mollier (24, 25, 26), Harrison (19), Wiedersheim (36), A. Smith Woodward (37), and Dean (9) have written in favour of the lateral fold theory; Davidoff (8), Fürbringer (12), Braus (3—7), and others have supported its rival. It is unnecessary for me in these notes to give a history of the discussions to which the question has given rise; the literature has been recently reviewed by Mollier and Braus, and the whole subject is familiar to zoologists. But there are certain essential points which seem to be in danger of being obscured from view in a cloud of controversy, and it is in the hope of clearing up some of these points and of filling up certain gaps in the evidence that these notes have been published.

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