viii PREFACE. 



Professor Hebert and his assistant M. Munier-Chalmas of the Sorbonne, 

 Paris, were equally kind and liberal. I desire also to thank M. Collenofc, 

 M. Breon, and Dr. Bochard, for their kind attention and the free use of 

 the collections at Sernur. Professor Owen and Dr. Henry Woodward of 

 the British Museum, Mr. Ethericlge of the Geological Museum, the authori- 

 ties of the Bristol Museum, and Dr. Thomas Wright, gave me similar 

 opportunities for study, and Mr. Marder at Lyme Regis assisted me in the 

 field. Prof. Jules Marcou has materially aided the work by the loan of 

 rare books not obtainable elsewhere, and I am also indebted to Prof. J. D. 

 Whitney for similar loans from his library. Professor Emerson of Amherst 

 has given me valuable information, and the use of his collection. I was 

 unfortunate in finding the curators of collections either absent or sick at 

 Hanover and Heidelberg; but in all practicable cases ample opportunities 

 for study were given me, except at the Museum of York, England, where 

 unyielding regulations prevented access to the interior of the cases, and my 

 identifications there were consequently made without handling the speci- 

 mens. I am also indebted to Professor Cope and Dr. John A. Ryder 

 for the results of investigations which have thrown much light upon 

 vexatious questions of theory, and which have not been pioperly repre- 

 sented by quotations in the text of this work, the general remarks having 

 been necessarily cut down to the narrowest possible limits. 



The essay on "Fossil Cephaiopods in the .Museum of Comparative Zoology" 1 

 was written in large part as an introduction to this monograph, but for obvious 

 reasons has not been used. The following conclusions, copied with some 

 emendations and corrections from that essay, may be useful, however, in giv- 

 ing the reader a view of the theoretical opinions entertained by the author. 



1. Law of Morphogenesis. — We have endeavored to demonstrate that a natural 

 classification may be made by means of a system of analysis in which the individual 

 is the unit of comparison, because its life in atl its phases, morphological and physio- 

 logical, healthy or pathological, embryo, larva, adolescent, adult, and old (ontogeny), 

 correlates with the morphological and physiological history of the group to which it 

 belongs (phylogeny). 



2. Organic Equivalence. — All new characteristics, even those which are purely 

 mechanical reactions of the tissues, arise in a similar manner, as reactions due to the 

 exciting agency of the more general or more localized physical causes. They are there- 

 fore necessarily, and because of this mode of origin, the corresponding organic, or suitable 

 complementary equivalents of these physical causes, both structurally and functionally. 



3. Alter their origin, however, and during their subsequent history, organic equiva- 

 lents or characteristics are divisible into two categories: those which become morpho- 

 logical equivalents, and are essentially similar in distinct series, and those which are 

 essentially different in distinct scries, and may be classed as morphological differentials. 



4. Morphological Equivalence. — In the different genetic series of a type derived from 

 one ancestral stock there is a perpetual recurrence of similar forms in similar succes- 

 sion, which are usually called representative and often falsely classified together, though 

 they really belong to divergent, genetic series. 



1 Pi-oo Am: Ass. Adv. Sci., XXXII., 18S3. 



