342 GEOLOGICAL BIOLOGY. 



vertebrata in the Paleozoic, and of the greater number of all 

 existing and fossil types before the expiration of Paleozoic 

 time, speak strongly for the quicker evolution of forms in the 

 Paleozoic, and indicate a general law of evolution. This, we 

 think, can be formulated as follows : Types are evolved more 

 quickly a7id exhibit greater struetural differences between ge- 

 netic groups of the same stock zvhile still near t lie point of origin, 

 than they do subsequently . The variations, or differences, may 

 take place quickly in the fundamental structural characteristics, 

 and even the embryos may become different when in the earliest 

 period, but, subsequently , only more superficial structures be- 

 come subject to great variations.'' * 



Summary. — If we ask, In what particulars does the structure 

 of Cephalopods illustrate this law? we may answer in brief, 

 that we notice it first in the class characters of the Cephalop- 

 oda. In the description of the class we found the Cepha- 

 lopods most closely allied to the Pteropods. This is con- 

 spicuously observed in the difference in structure of the 

 locomotor apparatus of the foot. In the Pteropod there are 

 two lateral flaps used like wings, or paddles, for locomotion. 

 The Cephalopods are modified to form a siphonal funnel 

 which accomplishes locomotion by forcing water violently out 

 and forward from this funnel ; other structural peculiarities 

 are associated with this modification. 



The Pteropods are abundant in the Cambrian faunas, and 

 appear to have attained a relative dominance never afterward 

 held, but in this first fauna there were no Cephalopods. The 

 Cephalopods of the next (Ordovician) period were extremely 

 abundant, and the Tetrabranchiata type was expressed by 17 

 of its 29 genera at the initial Ordovician stage (including here 

 the Upper Tremadoc, whose fauna seems more appropriately 

 associated with Ordovician than with Cambrian faunas). 



It is seen, secondly, when the Ammonoid type of the 

 Cephalopods made its appearance in the Goniatites. The 

 Goniatites came out in full force in the Devonian, with a few 

 species in beds doubtfully referred to the Upper Silurian but 

 called Lower Devonian by Kayser. The most characteristic 



* See "Phylogeny of an Acquired Characteristic," by Alpheus Hyatt, Proc^ 

 Phil. Soc, vol. XXXII., No. 143. P- 37 1- 



