PREFACE 



THE following papers from the publications of the Labora- 

 tory of Paleontology have been selected for reprinting on 

 account of their representing more or less closely a distinct 

 line of research; namely, the investigation and study of the 

 development of a number of invertebrate animals. The gen- 

 eralizations resulting from such studies properly belong to 

 the province of organic evolution, while the detailed methods 

 pertain to the observation and interpretation of the stages of 

 growth and decline in the organism. 



Nearly all the subject-matter i^ based upon studies of the 

 remains of fossil animals, many of them coming from the 

 oldest known fossil-bearing rocks. In some instances material 

 representing living species has been introduced for comparison 

 and to illustrate further the problems under investigation. 



The first work done in America on the stages of growth 

 of fossil Brachiopoda was a memoir by the present writer, in 

 collaboration with Dr. John M. Clarke, on " The Development 

 of Some Silurian Brachiopoda," published by the University 

 of the State of New York. It seems fitting to include this, 

 in order to complete the work on the development of the 

 Brachiopoda carried on subsequently by the writer. Another 

 joint paper, written in connection with Mr. Charles Schuchert, 

 is also introduced. 



The first paper in the present collection, on " The Origin and 

 Significance of Spines," is an attempt to apply the general laws 

 of evolution in the study of a particular structure throughout 



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