ROBSON : MOLLUSCA AND GENETIC RESEARCH. 229 



In this context we may toncli very briefly upon Pelseneer's 

 criticisms (15). This is not a very welcome task, as all students of 

 the Mollusca will unite in recognizing their indebtedness to the 

 celebrated Belgian malacologist. But I cannot refrain from 

 expressing the opinion that Professor Pelseneer has failed to exercise 

 discrimination in his review of this subject. He adduces many 

 cases which he considers are not conformable to the concept of 

 unit characters and segregation. For reasons given above, however, 

 the observations cited by him are not admissible as evidence. 

 Furthermore, Professor Pelseneer appears to pin his faith to the 

 F2 3 : 1 ratio as evidence, and to disregard the well-knowu 

 modifications of that formula. Again, he is scornful as to certain 

 interpretations based upon imperfect dominance which he ranks 

 among " complications verbales ". Now imperfect dominance is 

 a great deal more than this, when one considers how well its action 

 may be tested ; and, even if it may not explain all the cases of 

 intermediacy, it cannot be lightly dismissed. 



Much might be said on the wide subject of the correlation of the 

 facts of adaptation, distribution, habits, and association with any 

 theory of evolution. The geneticist's point of view has been well 

 stated on this subject by Bateson (2), who draws inter alia upon 

 the facts recorded by Coutagne in his account of the 

 polymorphism of the Mollusca of France (6a). The distributional 

 studies of Mayer (14), Clulick (9), Crampton (7) afford little 

 evidence for the orthodox selectionist ; and as far as Mollusca 

 are concerned the evidence for a selective death-rate seem.s to 

 consist only of Weldon's earlier work upon Clausilia (18). His 

 later observations (19) were, however, negative. Other cases 

 less well worked out (Boycott (3), Colton (6)) have been put 

 on record, but these are scarcely conclusive. Although the 

 phenomena of adaptation are apparent everywhere in the animal 

 kingdom, it must be confessed that insufficient intensive study has 

 been devoted to the adaptive significance of specific characters among 

 the Mollusca. It is an incredible fact that since the publication of 

 " The Origin of Species " sixty years have elapsed without a general 

 concurrence of opinion on this subject being arrived at. And yet 

 every naturalist must be familiar with numerous cases where it is 

 hard to find an adaptive explanation of specific characters other 

 than the customary appeal to ignorance. There is room in the 

 study of the Mollusca for a great deal of observation and intensive 

 study of this question. It cannot be settled without a wealth of 

 observations made in the field upon habits, ecology, food, enemies, 

 etc., and in such matters the general biologist a\\^aits the verdict 

 of the field naturalist. It has been said time after time, but is as 

 true to-day as it was forty years ago, that our knowledge of animal 

 ecology and habits lags far behind our descriptive taxonomy. This 

 balance should be redressed. The experimental side of genetics, 



