THE DEVONIAN PERIOD 363 



lated, they developed independently, and after the lapse of 

 sufficient time became notably different in the several embay- 

 ments. As the sea later spread over the land these distinct 

 faunas invaded the interior region from different directions, 

 one from the northeast, another from the south, another 

 from the northwest, and so on. As the widening seas mingled, 

 the faunas were one by one brought into conflict, much as the 

 invasion of North America by the French and English brought 

 them into opposition in Canada and the Mississippi Valley 

 in the eighteenth century. Just as we now have a mixed 

 French-English people in Quebec, so the mingling of the 

 Hamilton fauna of the East with the McKenzie fauna of the 

 Northwest produced a mixed race in which the influence of 

 the Northwest immigrants was strongest, and left its stamp 

 on the result. The commingling of two marine faunas results 

 in something more than a mere mixture of the two. The 

 struggle between two faunas usually crowds into extinction 

 certain weaker members of each assemblage, and it often 

 results in the rapid rise of entirely new forms not found in 

 either of the original faunas. 



In late Devonian times the result of this succession of immi- 

 grations and intermixtures was a fairly cosmopolitan fauna 

 inhabiting the seas from Alabama to Alaska and having close 

 relations with the animals of distant China and Russia. 



Changed conditions of life. From what has already 

 been said of the Devonian faunas and their migrations it 

 will be readily inferred that the fossils are abundant and 

 locally well preserved. The same groups which were impor- 

 tant in the Silurian are represented also in the next period, 

 although with different relative standings. In the Silurian 

 the animals of the clear seas were almost the only forms 

 extensively preserved. Our best-known Devonian rocks are, 

 however, chiefly shales and sandstones, and so the fossils in 

 them tell us of the animals which frequented the mud banks 

 and the sandy shores rather than the clear, open sea. Con- 

 ditions which are congenial for one group of animals may be 



