JAMES PERRIN SMITH 13 



of the higher, but they express a permanent type of structure, so far as 

 these genera are concerned, and after reacliing maturity do not show a 

 tendency to attain higher phases of development, but thicken the shell 

 and cardinal process, absorb the deltidial plates and exhibit all the evi- 

 dences of senility." 



E. D. Cope,* too, has expressed himself clearly on this question: 

 "The acceleration in the assumption of a character, progressing more 

 rapidly than the same in another character, must soon produce, in a type 

 whose stages were once the exact parallel of a permanent lower form, the 

 condition of inexact parallelism. As all the more comprehensive groups 

 present this relation to each other, we are compelled to believe that accel- 

 eration has been the principle of their successive evolution during the 

 long ages of geologic time. Each type has, however, its day of suprem- 

 acy and perfection of organism, and a retrogression in these respects 

 has succeeded. This has, no doubt, followed a law the reverse of acceler- 

 ation, which has been called retardation. By the increasing slowness of 

 the growth of the individuals of a genus, and later assumption of the 

 characters of the latter, they would be successively lost.'' This state- 

 ment of Cope might apply equally well to unequal acceleration or "tele- 

 scoping" of characters, but in another part of the same work he gives a 

 clearer statement:* "Where characters which appear latest in embry- 

 onic history are lost, we have simple retardation, that is, the animal in 

 successive generations fails to grow up to the highest point of comple- 

 tion, falling further and further back, thus presenting an increasingly 

 slower growth in the special direction in question." 



Examples of arrest of development are very common among the Am- 

 monites, especially towards the end of the history of stocks. These, 

 naturally, are more common and better known in the Jurassic and Cre- 

 taceous, where the family history is not so well understood, and where it 

 is not possible to correlate the arrested stages with ancestral genera. 



Lecanites and Nannites, of the Triassic, are regarded by some au- 

 thors as cases of reversion by arrest of development, but the writer re- 

 gards them as fixed persistent types. Much better illustrations are found 

 in the great families, Tropitidie and Ceratitid«, of which the genealogy 

 is well known, and where the arrested stages may be compared with an- 

 tecedent genera in the same line. Among the Tropitidaj the development 



♦Origin of the Fittest, p. 142. 

 *0p. cit, p. 13. 



