228 PEOCEEDINGS OF THE MALACOLOGICAL SOCIETY. 



liortensis the latter considered that he had showed that self- 

 fertilization did not occur. He found, however, that snails separated 

 after copulation could reproduce themselves, even if isolated for 

 three years. He considered this was due to the persistence of the 

 spermotozoa derived from the original copulation in the vesicula 

 seminalis for that period. In a later work (13), however, he announced 

 that a probable case of self-fertilization had been observed ; and 

 Kiinkel (11) stated that he had actually observed the process. 

 There appears to be very little reason for doubting these observa- 

 tions, which, if they are finally endorsed, should be of great 

 importance in the study of genetics. The question of delayed 

 fertilization is, however, not finally disposed of ; and it is just 

 possible that certain anomalous cases such as those recorded by 

 Stelfox (16) and CoUiuge (5) may be due to this. 



The amount of experimental work done upon MoUusca that 

 satisfies the conditions of an orthodox genetic study is relatively 

 small. A great many observations recorded by Pelseneer (15), 

 some of them adduced by him as evidence against Mendelian heredity, 

 have been made that in one way or another fail to fulfil these con- 

 ditions. Uncertainty as to the actual parentage, failure to carry the 

 exjDcriments to the Fg generation and other factors all combine to 

 render these observations, otherwise of value, nugatory as evidence 

 for or against the occurrence of segregation. 



The experiments of Lang (12, 13, etc.), supplemented by Kleiner's 

 work (10), and cytological studies by Baltzer (1), are the most 

 important genetic researches upon Mollusca. Of almost equal 

 importance are the results of Stelfox (16, 17) ; while that of 

 Kiinkel (11) upon Ariori, though less extensive, deserves mention. 

 The earlier work of Collinge (5), criticized and given a Mendelian 

 interpretation by Cockerell (4), is also worthy of notice. 



It is impossible to give a detailed criticism of all this work. On 

 the whole one would say that it affords ample evidence of the presence 

 of unit characters, and segregation. There are several instances, 

 however, in which the meaning of anomalous ratios is not clear, and 

 the interpretation given by authors is not altogether satisfactory. 

 For example, I venture to think that Lang's (13, p. 2-55) explanation 

 of the proportions of j^ale-coloured banding in the Fj and F2 

 generations from (P) pale-banded X pale- banded H. hortensis is 

 not as satisfactory as some other interpretations of modified F^ and 

 F2 ratios. 



If the question were to be asked point-blank, " Do these results 

 endorse the geneticist's point of view or do they suggest that some 

 other mode of inheritance is operative ? " I think the answer would 

 be that in such cases as have been carefully worked out the evidence 

 favours the former hypothesis. If there are difficulties of interpreta- 

 tion, the subsecjuent history of such crosses does not in any case 

 favour the earlier conception of the nature of intermediates. 



