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The general principle is self-evident that the higher 

 the forms, the greater their divergence in structure, 

 and the earlier the period at which they were intro- 

 duced, the more difficult it becomes to account for 

 their evolution. 



So far as known fossils are concerned, they furnish 

 no evidence to show that Fishes were evolved from 

 invertebrates. The oldest remains are those of highly 

 organized fishes which differed from each other 

 widely in structure. These Fishes could not have 

 been evolved from any of the known invertebrates 

 which existed in the Primordial. The time was too 

 short for the production of the enormous advance in 

 structure involved in the evolution of these fishes. 



If we rely on known fossils as evidence, we would 

 be obliged to conclude that highly organized fishes 

 were suddenly introduced. The break in the sup- 

 posed chain of evolution between the invertebrates 

 and the highly organized vertebrates of the Lower 

 Silurian is one of the greatest in the whole geological 

 record. As already stated, the discovery which car- 

 ries the history of Fishes back to the lower Trenton, 

 does not narrow the gap between the two great and 

 widely divergent divisions of the animal kingdom. 

 The vast gulf between these structures must, I think, 

 remain unbridged except by the imagination. 



The following quotations from Le Conte and Hux- 

 ley, which were written when the oldest known fossil 

 remains of fishes were in the Upper Silurian, are still 

 applicable since the discovery of their remains in the 

 Trenton. 



Le Conte says: "But it is impossible to overlook 

 the comparative suddenness of the appearance of a 

 new class — fishes — and a new department — verte- 

 brates — of the animal kingdom. Observe that at the 

 horizon of appearance in the uppermost Silurian 



