126 VIGNETTES FROM INVISIBLE LIFE. 



teresting inquiry "how acquired," remains for 

 investigation. 



Presuming a common origin, why is one free 

 and revolving, and the other non-revolving and fixed 

 to a local abode ? 



The answer is suggested by the terms of the 

 inquiry, especially so if the difference of habitat 

 be included in it, for it is easy to conceive how 

 a comparatively slight difference in their sur- 

 roundings, caused by some accidental circumstance 

 in the first instance, may have separated the original 

 family, and given each a new environment, to which 

 it would endeavour to accommodate itself, and in 

 so doing a modification of structure or motion or 

 both (the one acting on and influencing the other), 

 would result. So an upheaval or depression of the 

 surface of the earth may have separated or cut off 

 Conochilus from its fellows, leaving it for ages in 

 comparatively still water. Under these circum- 

 stances a great advantage, or perhaps rather some . 

 compensation, would be gained if a free and re- 

 volving motion such as we find it to have 

 could be obtained. It is remarkable that this 

 revolving tendency exists in both Lacinularia and 



