566 TEST ACE A ATLANTIC A. 



region of the Atlantic, was ever broken up at all, we can hardly 

 even contemplate a disruption on a scale so gigantic except 

 through the medium of some catastrophe to which the various 

 processes of disintegration with which we are acquainted give us 

 no kind of clue, and offer no parallel. But if cataclysms, as 

 such, can be permitted to form a portion (whether at stated 

 intervals I need not stop to consider) of the geological record, it 

 is quite clear that the depression of certain tracts, and the up- 

 heaval of others, would produce an amount of disturbance in the 

 fauna which could not fail to shew itself in some way or 

 other which would afterwards become more or less decypherable ; 

 and I cannot conceive much difficulty in picturing the kind of 

 change which might be brought about by the isolation of a 

 cluster of individuals on a small rock, destined henceforth to 

 become the habitat of a race which would, we may feel well- 

 nigh certain, rapidly mature for itself some slight distinguishing 

 mark. I say ' rapidly,' (1) because the very fact of a great and 

 sudden alteration in the surrounding influences would almost 

 imply & corresponding one (however insignificant comparatively) 

 in the organisms which had been thus cut-off abruptly from 

 their fellows, ' a corresponding one," moreover, which there is 

 no reason to suspect might not be consummated in the course of 

 a few generations ; and (2) because there is the strongest evi- 

 dence for concluding that no modifying process, whether pro- 

 gressive or retrograde, is going on at the present moment, for it 

 has not made itself so much as appreciable since even the sub- 

 fossil period ; so that whatever trifling varieties, or departures 

 from a central type, are now to be traced, were, in all proba- 

 bility, brought about quickly, and as the mere natural result of 

 a change in the conditions of the respective areas which their 

 progenitors had severally overspread. 



Considering how unmistakeable the evidence is for the 

 variability (in this particular sense) of many of the Atlantic 

 types which we have lately been discussing, a ' variability ' so 

 decided that a slightly different phasis has been assumed, in 

 certain of the archipelagos, for nearly every separate island, and 

 sola ted rock, it may sound perhaps somewhat paradoxical to 

 peak, nevertheless, of their apparent freedom from further 

 c hange ; and yet if there is one fact more distinctly shadowed 

 forth than another, it is, without doubt, their present stability. 

 It may be perfectly true that, when viewed from a geological 

 standpoint, the various deposits in which the shells are found to 

 occur more or less subfossilized are comparatively recent ; but 

 since there is every reason to suspect that a vast change both in 

 the conditions and extent of the surrounding districts has been 

 brought about sinoe the epoch of their formation ( a change 



