A LIVING FOSSIL — SMITH 323 



groups. These are known as the Placoderms (clumsy "armor-plated 

 fishes"), the Marsipobranchs (jawless sucking-mouthed fishes), the 

 Selachians (fishes with cartilaginous skeletons), and Pisces (fishes 

 with bony skeletons). Many were experimental forms which found 

 competition too severe and so vanished. All of the first group 

 are extinct. They were too clumsy. There are a few miserable 

 remnants of the Marsipobranchs still alive today (hagfish and 

 lampreys). The Selachii are the sharks and rays which have re- 

 mained vigorous and numerous, and which are one of the great forces 

 in the waters of the earth. In the vast periods of time since their 

 ancestors first spread terror in prehistoric waters, the sharks have 

 changed perhaps less than any other creatures. Many became ex- 

 tinct, but the line was carried on by forms of vigor and activity. 

 Under Pisces are grouped the vast majority of living fishes, and a 

 number who have vanished, some of great significance in the ancestral 

 line. 



The immediate importance of this recent discovery lies in the 

 information it affords us about the developmental processes which 

 have led to the typical forms of fishes, and which have been the 

 subject of much research and speculation. This Coelacanthid speci- 

 men sheds a great deal of light upon many of those questions, since 

 for some reason parts of this fish are in a condition which may be 

 termed arrested metamorphosis. That is, it bears certain structures 

 which are in process of changing from one thing into another, but 

 the change has not gone to completion. Many of the outer bones 

 of the head in fishes are supposed to have been derived from scales. 

 Also the teeth in the jaws of fishes are believed to be merely scales 

 that have migrated inward and have been changed into tooth-bearing 

 structures. Fins are regarded as having originated from continuous 

 folds of skin developed as stabilizers along the long axis of the body. 

 On these and on many other points the present specimen affords a 

 great deal of important evidence. 



The main outline of the evolution of various types belonging to 

 the two chief groups of fishes is shown by the accompanying dia- 

 gram, which is not to scale. Branches which reach the line 1939 rep- 

 resent groups and forms which have survived to the present day. 

 The others represent extinct forms. The cross-hatched line shows 

 the addition to this scheme necessitated by the recent discovery. 



One of the great main branches of the evolutionary tree was the 

 group of the Crossopterygii (or fringe -finned). They were mostly 

 large active predaceous fishes that probably dominated the extensive 

 areas in which they occurred. Like most fishes they originated in 

 fresh water and later migrated to the sea. A large number of forms 

 developed, and were characterized by this peculiar "fringe-finned" 



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