LIFE IN PRIMARY PERIOD 233 



on each side of an axial ray. This arrangement distantly 

 resembles that of the rays in a branchial arch, and led the 

 celebrated anatomist Gegenbaur to the audacious supposition 

 that one of the arches of the fish's gills had become modified 

 both in form and function until it developed into a fin. 

 Unquestionably an organ can change both its function and its 

 form, but there must be some reason for this change. We 

 might perhaps admit the possibility of such a change in the 

 anterior fins close to the branchial cavity, but how could it 

 happen in connexion with the posterior fins, which are far 

 removed from this cavity ? and what should we say of 

 the unpaired fins, whose structure so closely resembles that 

 of the paired fins that Tristichopterus appears to carry on its 

 back a third fin similar to the pectoral fins ? The embryogeny 

 of the Elasmobranchs agrees with comparative anatomy in 

 showing that the fins were at first represented by four 

 longitudinal folds of the body wall extending along its whole 

 length, one dorsal, one ventral, and two lateral, the last 

 two forming what is called the patagium, and the two former 

 the diphycercal fin, which only exists in Marsipobranchs. 

 Each segment of the body still furnished these fins, in the 

 course of their development, with the same number of rays, 

 muscles, nerves, and blood vessels. At first continuous, 

 the folds were subsequently broken (p. 227) at the places that 

 bore the force of the backwash set up by the quick flexions 

 of the tail to the right and left in swimming. It has been 

 suggested that the apparent absence of the lateral fins in 

 certain Ostracoderms is due to the fact that these flattened 

 Fishes have preserved their patagium or developed it again, 

 as the Rays and the Torpedo-fish, which live the same kind 

 of life, have done to a certain extent — more in appearance 

 than reality. 



We now come to the beginning of the Carboniferous Era. 

 The gradual perfecting of their organism has made the Fishes 

 the dreaded enemies of the Gigantostraca and the Trilobites, 

 on whom their relatively greater size has destined them to 

 prey— and whom they have probably caused to disappear 

 on that account. So the Fishes now prepare to invade the 

 land. Many of them had already penetrated into fresh 

 waters, and, apparently, taken to their new surroundings, 



