346 Evolution and Adaptation 
the median line are alike, and at this time capable of forming 
adult structures, a simple change of the spiral from right to 
left might determine on which side of the middle line the 
mesodermal cell would lie, and its presence on one side rather 
than on the other might determine which side of the embryo 
would develop, and which would not. This possibility removes 
much of the mystery which may appear to surround a sudden 
change of this sort. 
It seems to me that we shall not go far wrong if we assume 
that it is largely a matter of indifference whether an individ- 
ual snail is a right-handed or a left-handed form, as far as its 
relation to the environment is concerned. One form would 
have as good a chance for existing as the other. If this is 
granted, we may conclude that, while in most species a per- 
fectly definite type is found, a right or a left spiral, yet 
neither the one nor the other has been acquired on account 
of its relation to the environment. This conclusion does not, 
of course, commit us in any way as to whether the spiral 
form of the visceral mass has been acquired in relation to 
the environment, but only to the view that, if a spiral form 
is to be produced, it is indifferent which way it turns. From 
the evolutionary point of view this conclusion is of some 
importance, since it indicates that one of the alternatives has 
been adopted and has become practically constant in most 
cases without selection having had anything to do with it. 
Somewhat similar conditions are found in the flounders 
and soles. As is well known, these fishes lie upon one side of 
the body on the bottom of the ocean. Some species, with the 
rarest exceptions to be mentioned in a moment, lie always on 
the right side, others on the left side. A few species are 
indifferently right or left. At rare intervals a left-sided form 
is found in a right-sided species, and conversely, a right-sided 
form in a left-sided species. In such cases the reversed type 
is as perfectly developed in all respects as the normal form, 
but with a complete reversal of its right and left sides, 
