218 RESEARCHES ON FUNGI 



suffering any harm. 1 In this respect these molluscs resemble the 

 larvae of the fungus gnats (Mycetophilidae) which are often to be 

 seen in large numbers mining their way through the flesh of the pileus, 

 but differ from house-flies, which quickly become stupefied when they 

 sip the hyphal sap, and from cats, dogs, man, and other carnivorous 

 mammals, to which the fungus may even prove fatal. Why the 

 toxic substances present in the Amanitae should be deadly to 

 some animals, such as house-flies and man, and harmless to others, 

 such as slugs and the larvae of certain fungus gnats, has never 

 yet been sufficiently explained ; but for a solution of the problem 

 we must look to the biochemist. 



It is possible that slugs and fungus gnats have been dependent 

 on fleshy fungi for food for a period of geological time amounting 

 to some tens of millions of years. 2 The immunity of these animals 

 to fungus toxins may therefore have been brought about by natural 

 selection, those individuals which suffered least from eating 

 poisonous fungi having survived and left descendants. On the 

 other hand, the house-fly, the cat, man, etc., which have come into 

 contact with fungi either not at all or only occasionally, have never 

 had a chance to become immune to the poisonous species. 



I have collected evidence which goes to prove that cows may 

 eat a considerable number of Amanita muscaria fruit-bodies with- 

 out suffering in any marked degree. It therefore seems probable 

 that herbivorous mammals, which, like slugs, in the course of evo- 

 lution have fed in an environment where poisonous fungi have 

 been available as food, are far less susceptible to fungus poisons 

 than carnivorous mammals which do not eat vegetables and there- 

 fore never come into contact with fungi. Cows, like slugs, may 

 have acquired their relative immunity to the toxins of the Amanitae, 

 etc., through the process of natural selection. 



Russula emetica, the juice of which is acrid to man, is evidently 

 regarded as a tit-bit by slugs, for in English woods one often finds 

 its pilei partially or completely destroyed by these animals. In 

 1909, I showed that, under experimental conditions, Limax maxi- 

 mus, Arion subfuscus, and Agriolimax agrestis all eat Russula 



1 A. H. R. Buller, these Researches, vol. i, 1909, p. 229. 

 1 Re fossil Mycetophilidae, ibid., pp. 19-20. 



