128 C. E. Beeclter — Origin and Significance of Spines. 



physiological and internal structures, and that later the changes 

 are mainly physical and peripheral. 



One explanation of this would be that the forces of the en- 

 vironment are at first freely transmitted and produce internal 

 modifications, and that later these characters become stable, 

 making the effects of the external stimuli apparent in the 

 superficial differentiation of the organisms. 



In any event, the modifications in function and structure are 

 followed by modifications in surface, showing that the more 

 important physiological and structural variations are the first 

 to be subjected to heredity and natural selection, which tend to 

 fix or hold them in check. Features of less functional impor- 

 tance, as peripheral characters, are the last to be controlled, 

 and therefore present the greatest diversity, while in this diver- 

 sity spinosity is the limit of progress. In order to be hereditable, 

 the modifications through the environment must have induced 

 correlative internal adjustments and changed forces which can 

 be transmitted to offspring, and they in turn reproduce the 

 specific modifications. 



For the purpose of illustrating these statements, the evolu- 

 tion of the Brachiopods and Trilobites will be taken. The 

 Brachiopods are divided into four orders, all of which appear 

 in the Lower Cambrian and continue to the present time. 

 Schuchert 64 states that "of the 49 families and subfamilies con- 

 stituting the class, 43 became differentiated in the Paleozoic, 

 and of these 30 disappeared with it" ; also, " of the 327 genera 

 now in use, 227 had their origin in Paleozoic seas, or nearly 70 

 per cent of the entire class." Throughout the Cambrian, " dif- 

 ferentiation was mainly of family importance." u Differentia- 

 tion is most rapid near the base of the older systems, and dimin- 

 ishes the force from the older to the younger geologic 

 divisions." The most rapid increase was in the Ordovician, 

 the culmination was in the Devonian, and the rapid decline 

 came with the Carboniferous. About six thousand species are 

 known, and of these probably not more than one hundred and 

 fifty are living. 



Similar data are derived from the Trilobites. This group 

 is found all through the Paleozoic, at the close of which it be- 

 came extinct. Two of the three orders are found in the 

 Lower Cambrian. The remaining order appeared just after 

 the close of the Cambrian in the early Ordovician, yet through 

 the whole of the remaining sediments not a single new ordi- 

 nal type was enveloped. When applied to a single order, 

 the same truth comes out. The order Proparia is one whose 

 entire history can be traced, extending from the Ordovician 

 through the Silurian and Devonian. All the families appear 

 in the Ordovician ; in fact not a single family type in this or 



