394 Causes and Course of Organic Evolution 



bility to fresh water, brackish, and salt water, and, unless it 

 can be sho\^Ti that this is a recently acquired capacity, it should 

 rather be accepted that this indicates a variation tendency 

 slowly developed in primitive life to suit varying environal 

 changes that the group became exposed to during the process 

 of evolution. 



Accepting the estimates and classificatory values of several 

 recent conchologists, the entire group of living MoUusca is 

 made up of 843 genera, 483 of which belong to Gastropoda, 

 2!27 to the Lamellibranchiata, 61 to the Amphineura, 11 to the 

 Scaphopoda, and 59 to the Cephalopoda. But of the above 

 total about 260 genera are either fresh-water or terrestrial 

 or include species that are such as well as others that are brack- 

 ish or marine. In view of the statements that have generally 

 been made this seemed a. rather startling as it was an unex- 

 pected result, when the writer first compared the genera statis- 

 tically and geographically. 



But, in trying to unravel the phylogeny of the above five 

 groups of the molluscs, zoologists have often accepted it that 

 not merely the fresh-water forms are derived from marine 

 ones, but even that the Gastropoda-pulmonata have also had 

 such an origin, and later formed a pulmonary sac accessory 

 to the branchial sac, and which ultimately replaced it. It is 

 necessary therefore that we try to reach a correct estimate 

 as to the possible origin of each of the five groups. It will 

 generally be accepted that all possess a free veliger stage, or 

 have descended from forms that possessed such. Now in 

 passing from this to the adult stage many reasons could be 

 adduced for regarding the Gastropoda as the most primitive 

 group, while several genera belonging to it are found in Cam- 

 brian rocks. But of these the soft-bodied genus Vaginula 

 seems to represent a simple and ancient line in which diffuse 

 tegumentary respiration was effected, in which the animal 

 and its interior organs were straight, and the hepatic as well 

 as reproductive systems are like those of many worms. A 

 second line of origin seems to have led to the testaceous pul- 

 monates in which pallial breathing was also retained, but in 



