426 THE CAMBRIAN. [bull. 81. 



accurate knowledge of the species and their life history, including that 

 of the genus and family to which they belong. Types of restricted 

 range and short duration are of little importance, except as expressing 

 the stage of development of their genus. The persistent, long-enduring 

 types, that range from formation to formation and even from group to 

 group, may be of service if their study and analysis have revealed 

 characters by which their relative stratigraphic position, in a given 

 area, may be determined. The evolution of genera is of greater value, 

 and the analyses of the development of specific types within them may 

 be used in the identification of horizons. 



Extended to groups of genera, families, or orders, correlation by the 

 known evolution of the faunas may be used in a general way. In the 

 presence of other reliable data it is of relatively small value. Only in 

 instances where there are no other data that can be made available will 

 it be really of practical service to the geologist. 



Even under the best conditions and circumstances great care must 

 be used, owing to the varying conditions caused by environment aud 

 the unequal evolution of organisms in different geographic areas or 

 basins. I do not wish to imply that this method of correlation should 

 be entirely overlooked or dismissed in the presence of more reliable 

 data. It can be used in connection with other methods in correlating 

 any formation or group of formations, but only in extreme cases should 

 it be considered the only factor. When used, the result obtained can 

 be considered as little more than provisional. 



Life history, — A careful study of the life history of the various ele- 

 ments of the faunas promises to yield data that may be important in 

 correlation. An illustration of this is afforded by Prof. H. S. Williams's 

 study of Spirifera Icevis. 1 This is a type that is represented by various 

 forms from the Niagara limestone of the Silurian to the summit of the 

 Devonian, that are evidently descended one from the other, although 

 they have been described under several specific names. At each ho- 

 rizon the variation from the initial form serves to locate its stratigraphic 

 horizon and thus its relative age and position in space and time. The 

 tracing of the migration of forms from one area to another and the 

 changes produced upon them during this migration are to be taken 

 into consideration. The theory of Prof. Alpheus Hyatt, 2 that there is 

 a cycle of forms in the geologic series and a corresponding cycle of varia- 

 tion in the species of that series and in the individuals of any one spe- 

 cies, may lead to practical results. He finds it to prevail very exten- 

 sively in the Cephalopoda, and it will undoubtedly be found to apply 

 to all the other zoologic groups. The conclusions obtained will be more 

 or less marred by the imperfections of the geologic and life record, but 

 I look forward to the time in the future when minute biologic studies 



1 The Life History of Spirifer lams, Hall. Ann. N. Y. Acad. Sciences, vol. 2, 1881, pp. 140-160, pi. xiv. 



2 Genera of Fossil Cephalopods. Proc. Boston Soc. Nat. Hist., vol. 22, 1883, pp. 253-265. Also, many 

 other papers by Professor Hyatt, referring to and describing the Cephalopoda. 



