344: PALEOZOIC SYSTEM OF ROCKS. 



tiation and specialization, and the Devonian fishes are an admirable 

 illustration of that law. The law may be stated thus : The first intro- 

 duced of any class, order, or family, are not typical examples of their 

 class, order, etc., but connecting types — i. e., forms which, along with 

 their distinctive classic, ordinal, or family, characters, combine others 

 which connect them with other classes, orders, etc. In the process of 

 evolution such connecting or generalized forms as a common stem are 

 separated into several specialized branches. Thus the Devonian fishes 

 were not typical fishes as we now know them, but Sauroids — i. e., along 

 with their distinctive fish characters they combined others which closely 

 allied them with amphibians and reptiles. They were the representa- 

 tives and progenitors of both classes ; from this common stem diverged 

 two branches, viz., typical fishes on the one hand, and amphibians and 

 reptiles on the other. Such connecting links with other classes or or- 

 ders are variously called connecting t}^pes, synthetic types, combining 

 types, comprehensive types, generalized types. We shall usually call 

 them generalized types, and their differentiated outcomes specialized 

 types. We shall find many such in the course of the history of the 

 organic kingdom. 



Suddenness of Appearances. — But it is impossible to overlook the 

 apparent suddenness of the appearance of a new class — Fishes — and a 

 new department — Vertebrates — of the animal kingdom. At a certain 

 horizon, and that without break by unconformity, and therefore with- 

 out notable loss of record, fishes appear in great numbers and variety. 

 It looks at first as if they came without progenitors. This apparent 

 suddenness, however, is greatly diminished by recent discoveries. 

 Fishes, few in number, small in size, and low in organization, have now 

 been found far down in the Upper Silurian. The gap is still great, but 

 will be made less and less by continued discovery. Nevertheless, in spite 

 of all this it is difficult to account for the enormous increase in the 

 number, size, and variety of fishes at the opening of the Devonian un- 

 less we admit paroxysms of more rapid movement in evolution — unless 

 we admit that, when the conditions are favorable, and the time is ripe 

 for a certain change, it takes place with exceptional rapidity, perhaps 

 in a few generations. 



Amphibians and reptiles have not yet been found in the Devonian. 

 Fishes, therefore, were the highest and most powerful animals then 

 living. They were the rulers of the Devonian seas. The previous 

 rulers, therefore — viz., Orthoceratites and Trilobites, according to a 

 necessary law in the struggle for life — diminish in number and size, 

 and seek safety in a subordinate position. 





