Page:Popular Science Monthly Volume 8.djvu/200

188 From a sac-like body, moving freely through the water, and thus exposed equally on both sides to the same environment, and therefore bisymmetrical, we may suppose that all mollusks have been derived. If such a free-moving body became fixed, unless as a stemmed Ascidian, its parts would be differently conditioned as to environment, and the side more favored would outgrow the other. As the first part of the snail's body to bend out of line with the axis is the intestinal canal, we infer that this bend occurred far back in the snail's ancestry. It occurs in the oyster. As the last organ to share the general twist resulting from unequal growth of the sides is the heart, we infer that displacement of this organ occurred later down in the history of the type. It does not occur in the oyster.



The first step toward a spiral-shelled gasteropod was taken in the first mollusk whose environment on one side was mud or rock, and on the other water. Difference of environment was the first factor in this series of evolutions. Only this would induce one-sidedness, and acting through long periods it might induce excessive one-sidedness. It might carry an oyster as far along in asymmetrical growth as the partially rolled-up oyster called Gryphæa. When asymmetry came to be of advantage to the animal. Natural Selection began and carried it to greater excess, with the aid of other factors for Nature is too