REPORT ON THE SCAPHOPODA AND GASTEROPODA V 



(4.) Great length of time naturally helps escape from these barriers, for in the lapse 

 of years accidents are likely to occur enabling species to evade difficulties which would 

 in ordinary circumstances prove insurmountable. Hence the occurrence of a living 

 species in a fossil state will always justify the expectation of its having a wide local 

 distribution, and vice versd. 



(5.) Where barriers of depth and temperature do not check distribution there seems 

 in ordinary circumstances no limit to universality of distribution. 



(6.) There actually are existing species whose distribution is universal, no barriers 

 having availed to stop their passage. 



(7.) There still is no trace even in these oldest and most widely distributed species 

 of essential lasting and progressive change. I do not wish to overpress this point, 

 presenting as it does merely negative evidence. I do not assert that there are no species 

 of Mollusca which have thus changed. I only say there are some, even many, of the 

 oldest and most widely distributed species, which have not done so, and that so far as 

 I have had opportunity of observation, no proof has reached me of progressive permanent 

 and essential change in Molluscan development. 



