g78 PROF. G. C. BOURNE OX THE [Nov. 17, 



■characteristic of freshwater shells — viz. relative lightness and 

 smoothness of shell and operculum due to the greater proportion 

 of oro-anic over calcareous material, and might therefore be 

 attributed to the direct influence of external conditions, — then, 

 wherever circumstances were favourable, marine forms ascended 

 rivers and as a result of changed conditions of life assumed the 

 characters which in our artificial systems of classification are 

 attributed to Paranerita (the tropical Neritina of previous authors). 

 I can see no ajn'iori objection to this supposition, for if evolution is 

 still going on within a group of animals as it has gone on i\\ past 

 times, marine Neritids must still be passing into estuaries, and 

 from estuaries into rivers, and as they change their conditions of 

 life so they must react to their surroundings and undergo modifi- 

 cations of structure. And as the organization of marine Neritids 

 is extremely similar in all parts of the world, and as the con- 

 ditions obtaining in rivers are also veiy similar, a similar 

 environment acting upon a similar organization must jDroduce 

 similar results. Let no one object that the environment does 

 not have a direct influence on the organism. It can be proved 

 that it has in certain Mollusca. Take a sample of oysters that 

 have been reared for two years, say, in the Scheide and another 

 sample reared for a similar period, say, in the Bay of Arcachon. 

 Their shells will have such distinct and easily lecognizable 

 characters that an experienced eye w'ill have no difiiculty in 

 identifying them. Take both samples and lay them down, sa,y, 

 at Whitstable, and leave them there for another two years. At 

 the end of that time the two samples will still be distinguishable 

 because of the characters of the first two years' growth. But in 

 the last two years' growth they will exactly lesemble each other, 

 and this new growth will have neither Scheide characteristics 

 nor Arcachon characteristics, but Whitstable characteristics 

 difiering from both the former. These facts are well known to 

 oyster-merchants, and I have personally verified them. They 

 are proof of the direct action of the environment on the grow- 

 ing shell, and if only conchological evidence were forthcoming, 

 I should be inclined to accept the supposition put foi-ward 

 above. Indeed, before I made a detailed study of the genital 

 organs, I thought that it was the best explanation of the problem, 

 but when I found that the females of Nerita were diaulic, and 

 those of Paranerita triaulic, and that there were parallel dif- 

 ferences in the male organs, the explanation no longer satisfied 

 me. It is inconceivable that such a structure as the ductus 

 enigmaticus could have been independently evolved several 

 times over. At the same time I think it probable that many of 

 the fluviatile Neritidse have been independently derived from 

 marine Neritidfe. and I have entered upon this discussion in the 

 hope that others will make an anatomical examination of species 

 from difierent localities and determine how far they diflier from 

 one another. An anatomical study of freshwater ISTeritidae from 

 the Atlantic seaboard is very much to be desired. 



