30 STUDIES IN EVOLUTION 



found at an early period, while the greatest specific differen- 

 tiation occurs at a later period. This shows that the results 

 of variation at first affect the 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 

 environment 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 importance, as peripheral characters, are the last 

 to be controlled, and therefore present the greatest diversity, 

 while in this diversity spinosity is the limit of progress. In 

 order to be hereditable, the modifications through the environ- 

 ment 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 Brachiopoda and Trilobita will be taken. The 

 Brachiopoda are divided into four orders, all of which appear 

 in the Lower Cambrian and continue to the present time. 

 Schuchert*^* states that "of the 49 families and subfamilies 

 constituting the class, 43 became differentiated in the Pale- 

 ozoic, 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, "differentiation was mainly of family importance." 

 "Differentiation is most rapid near the base of the older 

 systems, and diminishes 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 



