964 PRINCIPLES OF STRATIGRAPHY 



in a definite order and at a uniform rate. A certain percentage 

 of these individuals may, after a while, cease to develop character 

 (a) while characters (b) and (c) continue to develop. Later in 

 some of these, character (b) may cease to develop further and (c) 

 continue alone, while in others (c) ceases to develop and (b) 

 continues. In another portion of the original group character (b) 

 may cease to develop first and (a) and (c) continue, after which 

 character (a) may stop in some and (c) in others — the other char- 

 acter continuing. The combinations possible by this method will be 

 readily recognized and the number of different types — mutations, 

 varieties, or species, according to the rank to which they are ad- 

 mitted — will be readily seen. The possibilities of differentiation 

 will be further recognized when it is considered that the length 

 of time during which each character develops may also vary greatly. 

 Complete cessation of development of characters has been termed 

 genepistasis by Eimer, and the differential cessation heterepistasis. 



Acceleration and Retardation in Development (Tachygene- 



SIS AND BrADYGENESIS). 



Another principle which is of great importance in this connec- 

 tion, and which was first clearly recognized by Hyatt and by Cope, 

 is acceleration or tachygenesis.''' Instead of a uniform rate of de- 

 velopment some organisms may develop more rapidly and so are 

 able to reach a higher stage in development. Differential accelera- 

 tion may obtain between the different characters. Again, retarda- 

 tion (bradygenesis *), first recognized by Cope, may progressively 

 diminish the rate of development, so that certain individuals in 

 some or all of their characters may fall more and more behind the 

 normal rate of progress. 



Illustrations of Orthogenetic Development. Some of the most 

 satisfactory series, showing development in definite directions, have 

 been brought to light by the labors of paLxontologists. Such series 

 are especially well known among the ammonoids, a class of cephalo- 

 podous Mollusca which began its existence in the Devonic, culmi- 

 nated in the Jura, and had its last representatives in late Cretacic 

 time. 



Some of the earliest studies of the developmental changes of 

 this group were carried on by Alcide d'Orbigny (8), who recognized 

 a distinct succession in the form and ornamentation of the shell 



* From Tftxi^s = fast and /3po5i/s = slow, y^vecLi = birth. The term bradyge- 

 nesis was used by Grabau in 19 10 (16} as a complement of the term tachygenesis. 



