42 



GILBERT C. BOURNE. 



differ in a very small degree from those of the marine species 

 of the genus Nerita, abounding in the seas in which the 

 oceanic islands inhabited by the fresh-water Neritids are 

 placed, it did not seem an unwarrantable assumption that in 

 each locality the marine species had ascended from estuaries 

 into rivers (just as prawns do in so many parts of the tropics), 

 and had been similarly modified as a result of the fresh-water 

 environment. But when I found that the accessory genera- 

 tive organs of the fresh-water species from different localities 

 were always alike, and differed in the same direction and to 

 the same degree from the accessoi'y generative organs of the 

 marine species from the same localities, particularly in the 

 fact that the female gonaducts of the freshwater species are 

 always triaulic, whereas those of the marine species are diaulic, 

 I was no longer able to sustain the opinion that I had first 

 formed as to the possibility of the independent but similar 

 modification of the marine species in different parts of the 

 world. It seemed to me impossible that the triaulic condition 

 should have been evolved several times over. The problem, 

 however, is of the same kind as, though of less magnitude 

 than, that presented by the cusps of mammalian molar teeth. 

 If we can conceive the presence in the germ-plasm of Neritidae 

 of factors competent to produce the triaulic condition of the 

 genital ducts, but that the activity of these factors is only 

 excited by the co-operating action of other factors — in this 

 case by reduction of the salinity of the water — the detailed 

 resemblances between structures existing in animals living 

 so far apart but under similar conditions are susceptible of 

 explanation. 



A few years ago such an explanation would have been 

 inadmissible. But since Mendelian experiments have shown 

 that definite changes affecting parts of the organism in a 

 similar manner may require the co-operation of two or more 

 factors, and cannot be produced unless those factors are 

 brought together ; and since such experiments as those of 

 Stockard on Fuudulus have shown that a relatively slight 

 change in the salts dissolved in water may induce profound 



