EVOLUTION II3 
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 Pisana are dwarfed and 
stunted in comparison with those members of the 
8 
