Fish origins— fresh or salt water? 265 



essentially confined to the inferences which may be drawn IVoni such lew marginal 

 deposits, of dehaic and other coastal regions, which have alone survived. 



But while the rarity of ancient deposits of typically continental type makes it 

 difficult to estabUsh the presence of fishes in early fresh water, this very condition in 

 itself affords strong support to the fresh water hypothesis. A startling feature of early 

 fish history is the way in which, in late Silurian and Devonian times, one major group 

 after another appears on the scene quite suddenly and yet fully difTerentiated, with 

 little or no trace of ancestors in earlier deposits. The most reasonable explanation of 

 this remarkable situation is that the early evolution took place in inland waters, 

 and that our dearth of knowledge is due to the dearth of ancient continental sediments. 



Importance of negative evidence 



Seeing our newspapers filled with reports of murders, robberies, suicides and the 

 like, we sometimes lament the fact that the reader of such journals tends to gain the 

 impression that all men lead lives of violence, whereas in reality nearly all are peaceable 

 citizens leading humdrum lives that are not at all newsworthy. The case of the pre- 

 Devonian vertebrates is quite comparable. Proponents of a marine origin point 

 with pride to the occurrences of vertebrates in relatively small numbers in a limited 

 series of Silurian marine — or supposedly marine — deposits. What is not remarked is 

 the much more important fact that in the vast majority of Lower Palaeozoic marine 

 formations there is not the slightest trace of any vertebrate. Such marine deposits 

 are widespread and highly developed, and frequently carry abundant invertebrate 

 faunas — but no vertebrates. 



If the early vertebrates were marine, why are they not commonly found in such beds? 

 A variety of possible explanations has been advanced by proponents of marine origins 

 and may be briefly discussed, seriatim. (1) Since vertebrates are relatively rare in 

 marine beds, lack of specimens is due to the chances of random collecting. But this is 

 unsatisfactory; in all post-Silurian periods, vertebrates, although seldom common, 

 are nevertheless found in a great variety of marine beds. (2) Vertebrates did not evolve 

 until the time when we begin to find them in marine beds. But vertebrates are known 

 well down in the Ordovician. (3) Vertebrates were present earlier, but did not develop 

 a hard skeleton until late Silurian times. But the few known Ordovician vertebrates 

 were already well ossified. (4) Vertebrates were present in the ancient oceans, but 

 acquired a bony skeleton capable of preservation only when they invaded fresh waters. 

 This is a rather better argument. One thesis is that there might have arisen on entrance 

 to fresh waters some physiologic condition causing deposition of calcium salts in the 

 dermis. But the armour of ancient ostracoderms and placoderms is of varied and 

 complex patterns, not a mere random deposition of calcium, and later fishes show no 

 sign of physiologic discrimination between fresh and salt water as regards degree of 

 ossification. I have myself aided the argument somewhat by the suggestion (Rosier, 

 1933) that vertebrate armour was important in defence against eurypterid enemies. 

 Perhaps armour did arise on entrance into fresh water in connection with defence. 

 But unless we believe that bone arose independently a very considerable number of 

 times in the vertebrates, this does not aid the marine argument. And finally, under 

 this sub-topic : if it be argued that presence of bone and fresh water life are correlated 

 in early vertebrate history, claims that most Silurian vertebrates with armour were 

 marine must be abandoned—" one cannot eat one's cake and have it too '*. 



