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present dextral specimens, though these are very rare. 

 Bivalve shells, too, sometimes have the valves reversed, and 

 with them of course the soft parts of the animal. The curious 

 thing about these reversed varieties, at any rate among the 

 Gasteropoda, is that the peculiarity is not only transmitted 

 to the offspring, thus producing colonies of sinistral-shelled 

 forms, but that the localities in which one species is found 

 reversed, generally produce similar aberrations in allied 

 species also. At first sight it seems natural enough that a 

 peculiarity should occur in any district in all allied species, 

 rather than in any one alone ; but when we consider that this 

 is a variety that arises, and can only arise, in the earliest 

 stages of the embryo, and cannot be altered or influenced 

 by its external influences after its formation, it does seem 

 most remarkable that one spot on the Yorkshire coast should 

 produce reversed examples of two species of Helix, and that 

 a single lane near Bristol should be peculiar for the occurrence 

 in it of several sinistral examples both of Helix aspersa and 

 H. hortensis. 



Varieties of size arise generally from unusually favourable 

 or unfavourable conditions. Dwarfed varieties occur where 

 food is scanty and the conditions of life are bad ; and if you 

 want a good example of that you need only examine the 

 poorer parts of London, and observe the effects of ill-circum- 

 stances on our own species, how that the average height and 

 weight is less than normal, and monstrosities are common, 

 and the average age is less than thirty years ; not by any 

 means because they are fittest to survive so, but because the 

 circumstances are so ill-suited to the species, that although 

 they do not cease to live, they live only half a life, or less. 



So again, observe the pond-snails, Liinncea stagnalis, in a 

 little pond on Chislehurst Common, crowded together, lacking 

 food, so that one day when an old newspaper was blown into 

 the pond, they immediately set to and devoured it, for it was 

 the best food they could get. Tliese snails, what with the 

 crowding and the lack of food, are dwarfed, scarcely half the 

 normal size of the species, and some of them are distorted 

 into a curious scalariform monstrosity — analogous, I imagine, 



