FINS 55 



which there is a continuous fold of skin extending along the 

 back, round the tail, and forward along the belly as far as the 

 vent. We have already seen that the development of a par- 

 ticular organ in the individual fish frequently repeats to a 

 greater or lesser extent the evolutionary history of that organ, 

 and the development of the fins provides yet another example 

 of the light thrown on the past by the study of embryology. 

 There can be httle doubt that the median fins arose as such a 

 continuous fold of skin, which in course of time ^ became 

 strengthened by the development of slender supporting rods 

 or fin-rays. Still later, certain parts of this fold, not definitely 

 required, tended to degenerate and finally to disappear alto- 

 gether, leaving the separate dorsal, anal, and caudal fins.^ Since 

 the median fins are the only ones present in the primitive 

 Lampreys and their aUies, and since they appear first in em- 

 bryonic development, we may be justified in assuming that 

 their evolution preceded that of the paired fins, an assumption 

 which is largely confirmed by a study of fossil forms (Fig. 23). 



The origin of the paired fins is not quite so clear, and the 

 subject has been, and still is, a matter for some controversy. It 

 must be understood, therefore, that the following explanation, 

 while appearing to be that which best fits the facts provided 

 by a study of anatomy, embryology, and palaeontology, does 

 not meet with universal support. 



It seems probable that at some stage after the median fin- 

 fold had made its appearance a further advance was made by 

 the continuation of this fold forward towards the head. Since 

 the vent lay in its path, the original fold spHt into two portions, 

 each of which ran forwards and somewhat upwards along the 

 lower part of the side and ended just behind the gill-opening. 

 This hypothetical condition is illustrated in the accompanying 

 figure (Fig. 23A). No fossil fish has yet been discovered ex- 

 hibiting such complete lateral folds, nor is such a stage found 

 during the development of any living form. At the same time, 

 two or three very primitive Sharks are known in which the 

 condition of the fins may be described as providing an inter- 

 mediate stage. In a form known as Cladoselache, for example, 

 the paired fins are lappet-shaped, like the dorsal and anal, with 

 broad bases, and their position strongly suggests that they have 

 originated from once continuous lateral folds. In Climatias, 

 another archaic species, both pectoral and pelvic fins are 

 preceded by a strong spine, and there is also a row of similar 

 spines along the sides of the body between these firis. As in the 



