176 PRIMITIVE FORMS OF LIFE 



walk by means of their pectoral fins, which present, so far as 

 this goes, no special modifications of the normal organ. The 

 Red Gurnet and Red Mullet walk on the sand by means of 

 three of the anterior rays of their pectoral fins, which have 

 become free and move like fingers. Frog-fish and analogous 

 fishes also make use of their pectoral fins for walking on sand, but 

 here we find a curious phenomenon of parallelism ; the portion 

 of the pectoral fins corresponding to the secondary rays is 

 attached as though it were a hand to a kind of arm supported 

 by two primary rays resembling the radius and the ulna, and 

 which themselves are mobile, and move on an unpaired element 

 resembling a humerus. There is, be it understood, no 

 genealogical connexion between the pseudo arm of the Frog- 

 fish and the anterior leg of the Batrachians, but the fact that 

 a similar development could have taken place so much later 

 at the expense of a fin already highly modified shows that it 

 might have taken place also at the expense of primitive fins 

 under the influence of the same mechanical conditions. 



Unfortunately, the Dipnoan fishes, so closely related to the 

 Batrachians in many ways, have only left us representatives, 

 either in living or fossil form, in which the fin-skeleton is an 

 axis consisting of pieces placed end to end. They remain fairly 

 .simple in Protopterus and Lepidosiren, but in Ceratodus they 

 bear a double pennate series of multi-articulate rays. From 

 these facts we cannot draw any inferences as to the origin of 

 teet. We may feel fairly certain, however, that they are derived 

 from fins. Indeed, all the Fish and all the higher Vertebrates 

 are born with their limbs. Only the Batrachians are born 

 without them, and do not acquire them for a long time. Their 

 feet are formed slowly, in a very special way, and no longer as 

 accessories of the muscular segments or myotomes of the body 

 of the embryo, as in the case of all the other Vertebrates, but 

 as developments of small internal, isolated buds. The anterior 

 feet remain for a long time concealed under the skin in Frogs, 

 Toads, and other tailless Batrachians. This delay in the 

 appearance of the limbs can be explained if we assume that it 

 corresponds to a period in which the fins that the Batrachians 

 took from their ancestors, the Fish, are reabsorbed, after being 

 normally formed, in order to be replaced by feet. As required 

 by tachygenesis, to the normal period in which the fins 

 became gradually transformed into feet without ceasing to 



