8()4 PROF. G. C. BOURNE ON THE 



preserved. Terrestrial pulmoiiates aj-e still scanty in the Permian 

 and Trias, and only begin to show a considerable increase in the 

 Jurassic and Ci'etaceous. I need not labour the point further. 

 Clearly, palteontological evidence does not favour Dr. Sim roth's 

 theory of the orgin of marine from terrestiial Gastropoda. 



But let us suppose that palteontological evidence may be ignored 

 on account of the imperfection of the geological record, and tha.t 

 the Pendulation theory is so well supported by other evidence as 

 to compel us to give credence to Dr. Simroth's doctrines as to the 

 origin of marine from terrestrial Gastropods. The Helicinidre 

 are terrestrial and pulmonate. I have shown, and in so doing- 

 have only corroborated the opinion of all other observei'S, that 

 they are Neritoid in almost every feature of their anatomy. If 

 the marine and iluviatile Neritids were to be derived from a ter- 

 restrial and pulmonate form, one would suppose that that form 

 must have been Helicinid in character, for the affinities between 

 the two groups are so very obvious. But Dr. Simroth does not 

 discuss this possibility. Making refei'ence to Ostracolethe^ 

 Hycdimax, Limax, and Arion, all highly specialized recent 

 Pulmonates, he boldly derives the Neritida^ from the Stylomma- 

 tophora, relying largely upon the supposed homology of their 

 generative ducts. This homology I do not admit : a resemblance 

 there is, but not a close one, and, even if it were closer than it 

 actually is, I should place very little reliance on the anatomy of 

 the generative ducts as indicative of relationship between groups 

 differing widely in all other respects. In the different phyla of 

 invertebrated animals the generative ducts are notoriously variable 

 in character. In the Platyhelmia., for example, their variety is 

 bewildering. Within the phylum Mollusca there ai'e many in- 

 stances of variability and also of deviations which must have been 

 independently acquired but are in the same dii-ection, as, for 

 instance, in the Doi-idomorpha and Elysiomorpha,. The re- 

 semblances, such as they are, between the generative ducts of the 

 monoecious Pulmonata and the direcious Neritidfe are just what 

 one might expect to find in animals in which a common plan of 

 organization, to wit a gastropod organization, is modified in 

 accordance with similar physiological requirements. The differ- 

 ences are of amply sufficient niagnitude to betray a difference of 

 origin. In other words, the complex gonaducts of Neritida^ and 

 Pulmonata are independently acquired structures, and such 

 resemblances as they display ai'e due to pai'allelism. 



I have already referred to the anatomy of the Pleurotomariidas, 

 a family which existed in the Cambrian and survives to the present 

 day. Thanks to Bouvier and M. F. Woodward, we are well 

 acquainted with the anatomy of Pleurotomaria, which affords a 

 striking confirmation of the reliability of sound morphological 

 reasoning. Before Pleurotomaria had been studied, comparative 

 anatomists, as the result of extensive investigations of gastropod 

 structure, had come to an agreement concerning numerous marks 

 of primitive organization in the gioup. When this survivor from 



