420 JAMES P ERR IN SMITH 



is usually constant in the stages it goes through, it often happens 

 that the individual is arrested in development, never reaching 

 the full generic development of the mature stage. The individ- 

 ual then begins to reproduce its kind before maturity is reached, 

 and tends to give rise to a stock that never reaches the full 

 generic evolution of its ancestors. Dr. C. E. Beecher has well 

 described this: "In each line of progression in the Terebra- 

 tellidse the acceleration of the period of reproduction, by influ- 

 ence of environment, threw off genera which did not go through 

 the complete series of metamorphoses, but are otherwise fully 

 adult, and even may show reversional tendencies due to old age ; 

 so that nearly every stage passed through by the higher genera 

 has a fixed representative in a lower genus. Moreover, the 

 lower genera are not merely equivalent to, or in exact parallelism 

 with, the early stages of the higher, but they express a perma- 

 nent type of structure, so far as these genera are concerned, and 

 after reaching 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 evidences 

 of senility. 1 



If, then, the morphologist tries to study the race history in 

 one of these species thus arrested in development, he cannot 

 read the whole story, for the individual ontogeny will not reca- 

 pitulate the higher stages lost by retardation. 



Another remarkable case is that of the so-called "ceratites" 

 of the Cretaceous. While there have been no goniatites since the 

 Paleozoic, and no ceratites since the Trias, there are found among 

 the ammonites of the Cretaceous some with septa of simple 

 goniatitic character, and others with septa like those of the 

 genuine ceratites. Now since the line of descent is broken, and 

 there is no possibility for a continuous line of these ancient 

 primitive forms to have bridged over the great gap from the 

 Trias to the Upper Cretaceous, we must explain this either by 

 reversion or in some other way. But it is not a simple case 

 of reversion, for, as has been pointed out by several writers, 



1 Amer. Nat., Vol. XXVII, 1893, P- 6°3- 



