HISTORICAL SUMMARY. 11 



same careful and diligent observers— that of the comparison of large numbers of individuals 

 presenting gradational modifications of the same fundamental types; a method first applied by 

 Prof. Williamson to La^ena, and afterwards by myself to OrhitoUtes, Orhiculina, Peneroplis, 

 Opercidina, and Calcarina. The particular results to which they have been thus conducted 

 will be detailed in their proper place ; at present it will be sufficient to state that they have 

 been led most fully to accept the views of Prof. Williamson and myself in regard to the wide 

 range of variation which prevails in the group of Foraminifera generally, and that they have 

 given to those views a most remarkable development (lxxv — lxxx). The same observers are 

 engaged upon an extensive comparison of the Foraminiferous Faunas of different localities and 

 epochs, with a view to elucidate the conditions that determine the prevalence of particular 

 types. Of this comparison the results already given to the world are replete with interest 

 (liv, lv, lxxxi). 



