Miss Haviland, The bionomics of Aphis grossulariae, etc. 269 



A. grossulariae is not the natural summer form of A. viburni, but 

 is merely a casual parasite of the currant. In those of the Aphidinae 

 which have a regular migration between two plants, the change is 

 usually from a woody stemmed primary, to a herbaceous secondary, 

 host; and if in the case of ^. viburni, the currant should be found 

 to be the normal second host, it would be a remarkable exception to 

 this rule. Perhaps we have here a form that has not yet adapted 

 itself to the conditions of modern fruit growing. In a natural state, 

 the aphides are probably able to follow the whole life cycle on 

 Viburnum, but the spread of the cultivated currant has presented 

 them with an increasing supply of alternative food which induces 

 a change that makes a return to Viburnum impossible. Whether 

 sex-producing forms can arise from the currant stock, and thence 

 return to the guelder rose, is not known. If not, and the early date 

 of the disappearance from the currant is against this view, we must 

 consider that the infestation of the currant is an unfortunate 

 accident in the history of the species, which entails a waste of 

 migrating individuals upon a cultivated plant that might otherwise 

 have perpetuated themselves on the natural host. However this 

 does not mitigate the danger of the pest from a fruit grower's point 

 of view, and infected Viburnum ought not to be allowed in the 

 neighbourhood of currant bushes. 



Note on an experiment dealing with mutation in bacteria. By 

 L. DoNCASTER, Sc.D., King's College. 



[Read 17 February 1919.] 



(Abstract.) 



It was noticed that the recorded ratio of occurrence in cases of 

 meningitis of the four agglutination-types of Meningococcus corre- 

 sponded very closely with the ratio of occurrence of the four iso- 

 agglutinin groups of blood in a normal human population. It 

 seemed possible, therefore, that by growing Meningococcus of one 

 type in media containing human blood of ditferent groups, mutation 

 to other types might be induced. Experiment showed that con- 

 siderable differences in type of agglutination resulted, but it was 

 concluded that this was caused by the sorting out of races of 

 different agglutinability from a mass culture, rather than by true 

 mutation. 



