EVOLUTION 113 



on their sides, may be attributable to a like cause, 

 is not clear, but it is at least remarkable that so 

 many of the Monomyaria should be forms that 

 assume a position out of the normal vertical. 



Tridacna, so long a puzzle, and concerning which 

 it was even held that the animal must have rotated 

 in its shell, has been shown to be simply a case of 

 a Monomyarian that has taken to live with its umbo 

 downwards. All its anatomical features correspond 

 closely in arrangement and position with those of 

 Mytilus, only it occupies a relatively reverse position 

 and its huge plastic body tends by its own weight 

 to spread out, and consequently to form a shell that 

 has its longer axis in a direction at right angles to 

 that in the Mytilus shell. 



Among other variations induced by environment 

 is that of " dwarfing," which in every case appears, 

 and naturally so, to be traceable to the prevalence of 

 unfavourable conditions. 



Thus there is a tendency in Land Snails to become 

 stunted in size, as they attain to altitudes beyond 

 those in which food is plentiful and other con- 

 ditions of life most favourable to them. This is 

 noticeable among forms dwelling in high valleys of 

 the Alps, and is observable even in those on high 

 hills in our own country. Indeed it is claimed that 

 on Giltar Head, Tenby, which is 100 feet above sea- 

 level, the specimens of Helix Piscina are dwarfed and 

 stunted in comparison with those members of the 

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