I) 
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Vill PREFACE. 
Professor Hébert and his assistant M. Munier-Chalmas of the Sorbonne, 
Paris, were equally kind and liberal. I desire also to thank M. Collenot, 
M. Bréon, and Dr. Bochard;, for their kind attention and the free use of 
the collections at Semur. Professor Owen and Dr. Henry Woodward of 
the British Museum, Mr. Etheridge 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 properly 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 Cephalopods in the Museum of Comparative Zodlogy”’ 
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 
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 all 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. Organie Hquivalence.— 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. After 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 series, 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 succcs- 
sion, which are usually called representative and often falsely classified together, though 
they really belong to divergent, genetic series. 
1 Proc. Am. Ass. Adv. Sci., XXXII., 1888. 
