﻿375 



of the habitat is the working agent of all the major changes ; first 

 taking effect as a rule upon the adult stages, and then through 

 heredity upon the earlier stages in successive generations. Thus 

 in the open fields of the periods of their origin they expanded into 

 their different habitats, varying to accomplish this purpose with 

 great rapidity, but once in their appropriate habitat, inducements 

 to change or open fields became rarer and we get as a result com- 

 parative invariability. As time rolled on and the earth became 

 more crowded, the variability was reduced to less and less import- 

 ant structural changes, except in the retrogressive types. These 

 exceptions are our best proofs of the action of the habitat. The 

 changes in these retrograde forms are again' remarkable for 

 the rapidity in which they take place, and some of these types, at 

 least, can be shown to have occupied free fields where they met 

 with new conditions, and to have changed their habits and struc- 

 tures rapidly to accord with these new conditions. 



In 1843 Auguste Quenstedt began researches which ought long 

 ago to have led to this solution. He demonstrated by repeated ex- 

 amples, that among diseased types the most extensive changes of 

 form and structure might take place in a single species, and within 

 the narrowest limits of time and surface distribution. Quenstedt 

 was thus the first to show that in diseased forms the shell had the 

 inherent habit of reversing the process of growth and evolution, and 

 of becoming more and more uncoiled by successive retrograde 

 steps. Von Buch and Quenstedt, master and disciple, and the 

 author independently of either of these predecessors, in three suc- 

 cessive researches, have arrived at the identical conclusion, that 

 these uncoiled shells are truly distorted, or, as we may more 

 accurately express it, pathological forms. They are not, however, 

 rare or exceptional, as one might at first suppose, but occur in num- 

 bers and in every grade, from those that differ but little from the 

 normal forms, to those that differ greatly ; from those that are ex- 

 ceedingly confined in distribution, to those which lived through 

 greater lengths of time. But in all cases they exhibit degradation, 

 and are expiring types. The author has repeatedly traced series of 

 them, and studied their young, partly in Quenstedt's own collec- 

 tion. In all cases they show us that great changes of form and 

 structure may take place suddenly j and this lesson could have been 

 learned from Quenstedt's work and example as well forty years since 

 as now. 



