SUMMARY. 567 



so radical as to lead to the belief that some of them were pro- 

 bably matured previous to the break ing-up of the intervening 

 land), we have at any rate a monstrous period at our disposal 

 from which to judge whether any modifications have been 

 effected in the outward contour of the several forms ; and, after 

 the most rigid and conscientious enquiry, I am bound to add 

 that the ' developments,' so-called, which might well be sup- 

 posed to have been slowly elaborated, are (if any) simply inap- 

 preciable. Here and there a species may present itself which 

 would seem to have degenerated as regards the mere size of the 

 individuals which compose it ; but there is nothing to warrant 

 the idea of any gradually advancing movement (however in- 

 finitesimal) ; and indeed even in the few instances to which I 

 have just called attention, it is fairly open for enquiry whether 

 the two aspects of the shell were not, after all, contempo- 

 raneous, — the smaller one having only taken the place of the 

 larger, in point of individual numbers, in more modern times. 



It would appear therefore as if we were driven to the con- 

 clusion that these numerous phases of certain central, dominant 

 types (such as are observable, for instance, in the sections Dis- 

 cula, Coronaria, and Leptaxis) were brought about in the 

 comparatively remote past, and in obedience to circumstances 

 which may or may not have been exceptional, but which never- 

 theless answer better to what are commonly called ' catas- 

 trophes ' (though perhaps wrongly so) than to anything else. 

 And here it will immediately be perceived how the doctrine of 

 excessive segregation dovetails-in (as it were) with that of the 

 sudden breaking-up of a more or less continuous tract and the 

 rapid after-elaboration of colonies which may be termed ' in- 

 sular,' and which are characterized (to a greater or less extent) 

 by certain distinctive marks too often looked upon as neces- 

 sarily specific ones. 



But these and kindred problems are so purely speculative 

 that it will perhaps suffice to have merely glanced at one or 

 two of them, — in order to direct attention to the kind of evi- 

 dence which may hereafter prove to have been an unsuspected, 

 but nevertheless appreciable, item towards their solution. 



