182 Report of the State Geologist. 



individuals, but the entire species, and come thus under the law 

 of acceleration. They appear at first sporadic, and seem related 

 to pathologic phenomena ; they indicate a change unfavorable to 

 the average conditions. But presently, in formations imme- 

 diately above, the geratologic alterations become very frequent^ 

 which is explicable by the fact that the same physical modifica- 

 tions will produce identical eftects on animals of the same 

 species {law of morphogenetio equivalence of Hyatt). The gera- 

 tologic form, at first exceptional, thus becomes normal, and shows a 

 constant tendency to manifest itself earlier ; thus a distinct species 

 is established. The evolution of fixed forms or parasites seems to 

 find its explanation in this phenomenon. But it is particularly 

 striking in the Cephalopoda, where it has undergone a thorough 

 investigation. It is seen both in the Tetrabranchiata in the Silu- 

 rian, and among the Ammonitidae, in the course of the secondary 

 period. From it are deduced important conclusions concerning 

 the evolution of the group. 



In general, forms thus modified are affected with a sort of con- 

 genital weakness which renders them less fit for the struggle for 

 life, and deprives them of any long line of descendants ; this 

 occurs in the uncoiled Cephalopoda, which attain considerable 

 magnitude, and then suddenly disappear ; this phenomenon is 

 observable at diverse epochs and at the expense of distinct 

 groups. It is especially noticeable during the Cretaceous period. 

 It seems, at the close of that period, as though the entire group 

 had been affected with some malady ; the unrolled forms multi- 

 ply ; the septa are simplified and, frequently present the appear- 

 ance of those of the Ceratites of the Trias; the ornaments 

 become heavy, thick, and often disappear in the adult, and the 

 entire group ends by dying out and leaving no descendants.- 



But this is not always the case. Geratologic phenomena 

 sometimes produce simplifications not always unfavorable to the 

 species ; they may cause a convergence toward the primitive forms 

 of the group, and the types thus constituted may in turn be the 

 point of departure for new series in regard to which they will 

 hold the relation of primitive forms or radicles. In this event 

 the law of embryogenic acceleration will strongly come in force. 

 All the stages which have led to the constitution of the new 

 radicle form will be passed through with a celerity proportionate 



