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. 



