LIFE ON THE EARTH. 113 



The freshwater Mollusca altogether appear of 

 later origin than the marine tribes, and the Gaste- 

 ropoda appear to be of later origin than the Dimy- 

 aria, in fresh waters as well as in the sea. If we 

 compare the fossil and living Paludinse, Physse, Lim- 

 nseae and Planorbes, we are struck by the great re- 

 semblance in each case; Physa in each case has the 

 sinistral spire, Planorbis is discoidally depressed, 

 Paludina rises in tumid whorls, Limnsea extends 

 from a large aperture to an acute point. Here 

 again is the evidence of long duration of even slight 

 peculiarities, persistence of type, and restraint of 

 variation. 



If in either of these cases the Unionidse the 

 Paludinadse the Limnseadse Planorbes Phy sae, 

 &c., the modern forms are derived from the ancient, 

 we have the full measure of the whole variation 

 the differentials of change are all integrated by time, 

 and we behold the sum how little! But if not so, 

 if the modern and ancient species have sprung from 

 different branches of a stem still older than either, 

 how much stronger, if possible, is this decisive tes- 

 timony against the doctrine of indefinite change 

 through time and circumstance ! Circumstances have 

 varied, ages have passed away, and yet every generic 

 group exhibits at every step the same essential cha- 

 racters, and many of the little peculiarities, such as 



K. L. I 



