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THE MOLLUSCA AS MATEEIAL FOR GENETIC RESEARCH. 

 By Guy C. Eobson, B.A., F.Z.S. 



Read 10th June, 1921. 



In the following pages I wish to comment upon some of the observa- 

 tions and experiments which have been made upon Mollusca that 

 are of importance in the study of genetics. The advantage of such 

 a survey, limited as it is to one group of animals, may not be 

 apparent at first sight. The phenomena of segregation are now 

 known to be almost universal among animals ; but it will be, 

 nevertheless, of advantage to know whether certain groups show 

 peculiar types of segregation ; whether there are special problems to 

 be studied in certain groups ; and whether a special technique is 

 required for certain cases. Co-operation between the taxonomist 

 and geneticist should thus prove to be of advantage. 



The prominence given by authoritative workers to the selection 

 hypothesis and to the discovery of unit characters and segregation 

 has had the unfortunate result of making the average naturalist 

 consider that these questions are settled one way or another, or, as 

 an alternative, that both may be true. The phenomena of segrega- 

 tion and unit characters are almost universal, the number of instances 

 of well-attested selective death-rates and the clear demonstration of 

 natural selection at work are very few ; yet some of the arguments 

 adduced in favour of the latter hypothesis remain unrefuted, and 

 many phenomena of heredity are only brought under a Mendelian 

 interpretation by dint of considerable straining. There is room, 

 therefore, for more work of an experimental character and great 

 need for field naturalists to carry out supplementary observations 

 according to systematic plans. 



At first sight the Mollusca should be a highly satisfactory group 

 for experiment. The shell is a sensitive index of genetic change, 

 albeit susceptible to " fluctuating " variation. It is a permanent 

 and easily visible source of reference. Furthermore, there are certain 

 internal structures (the radula, the dart and jaws of Pulmonata, 

 the mandibles of certain Prosobranchs, and the stomachal plates of 

 Opisthobranchs), which afford admirable material for correlation 

 with the shell characters. On the other hand, they are not quick- 

 breeding animals, the land forms do not have very extensive 

 families, and, though otherwise well suited for study, by reason of 

 their moncecious sex, peculiar copulatory habits, and the tendency 

 among some to bury their eggs, are animals in which the business 

 of exact affiliation is a troublesome matter. We have lastly to 

 consider in detail a question raised by Lang's. original work (12) ^ 

 upon Helicidee. In his first report upon crosses of //. nemoralis and 



^ Numbers in ( ) refer to the bibliography at the end of the paper. 



