PREFACE. 
of organic change in this direction. While discussing certain generic and 
specific forms as characterizing known geological horizons or certain groups of 
strata, we had not yet taken into consideration the fact that modifications of 
organic types had been coincident with every change or progress in geological 
time. The great law of progress through long intervals had been everywhere 
recognized in geological science, but just how or in what manner these changes 
had supervened had rarely been shown in detail. Certain fossil genera have 
long since been recognized as Silurian, others as Devonian, and others as 
Carboniferous, but these are never entirely restricted to the formations which 
they are said to characterize. They have all doubtless been derived from 
some remote progenitor, and at certain horizons, or throughout certain forma¬ 
tions have become so abundant and so fully developed, that they are said to 
characterize that stage or formation. The most abundant and extravagant 
forms among fossil organisms can usually be traced to some parent stock of 
more modest pretensions, and in their early appearance, represented by few 
individuals. 
As stated, the studies of the Brachiopoda to the close of Volume IV of the 
Palaeontology had shown the importance of some investigation which should 
deal directly with these questions. And moreover the science demanded the 
results of such an investigation in aid of its future progress. 
The original conception and plan of the work which the author had 
proposed to himself was a very simple one, viz.: to select the earliest 
representative of a genus in any of the geological formations and to follow it 
through all its manifestations and modifications in geological time, to its final 
disappearance; or so far as these modifications should appear in the Palaeozoic 
rocks, to which he had limited his research.* With the knowledge then 
possessed and with the collections at his disposal he had supposed that the 
result of such an investigation could be embraced in a supplementary part to 
Volume IV, and under this title the work was announced. This study was 
commenced very soon after the publication of that volume and its general plan 
*The difficulty of procuring- sufficiently abundant and characteristic collections of the later forma¬ 
tions was in itself a sufficient barrier, and the scope of the work did not contemplate the discussion of 
Mesozoic and later genera, except in an incidental manner. 
