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PROGRESSION AND RETROGRESSION.

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rich to be limited to one or two efficient causes—carried it to the order of Gasteropods.

In this order we find Eolis, and Doris, and Aplysia. From them our studies have ranged over kindred, near and remote. From their kindred we return, prepared by what we have found to interpret them. In form, these animals do not depart from bilateral symmetry, as from their habits they should not. Each side is exposed in the same way to the same environing element. But the alimentary canal is bent out of line with the axis of the body. The reproductive system is still more askance. It is altogether one-sided. Very suggestive facts. We find one-sided growth without the conditions which induce it. These conditions must have pertained to an ancestor. The bend in the alimentary canal and the displacement of the reproductive organs have been inherited from an ancestor so conditioned in the environment as to produce overgrowth of one side. But the alimentary canal does not bend out of line so much as in the shell-bearing Gasteropods; and in Eolis—in which the last vestige of a shell has disappeared—the canal has become straight. Another suggestive fact. We find in these naked mollusks heredity and abbreviated heredity.^[1] Aplysia and Doris inherit the ancestral twist. In Eolis the heritage is cut off.

From symmetry to asymmetry, from a bivalve to a univalve, Nature has moved, closing a cycle of evolution in the snail; from asymmetry back to symmetry, from a shell-bearer to a non-shell-bearer, she is moving in the sea-slugs. In this retrogression, Aplysia has shared the least. It retains the largest shell-vestige; it has the most perfect liver; its gills cover the mantle. Eolis has been carried back the farthest. In this retrogressive movement we may find the rationale of Aplysia's many stomachs, and Eolis's branching stomach and hepatic cells. In the snail, perhaps in all Gasteropods, the alimentary

↑ To account for the facts of heredity, Darwin has formulated a theory called Pangenesis. To account for the facts of heredity and abbreviated heredity Dr. Elsberg has proposed a theory which he calls "the Conservation of the Organic Molecule." The biologist must be allowed as much "scientific use of the imagination" as the physicist. If the one must have his atoms and molecules, the other must have his physiological units, his plastic molecules, his "plastidules." Let us imagine the first pair of any race, say the human race. A child of the Adam and Eve would be derived wholly from its parents, and, if the plastidules which passed into the embryo were derived equally from each parent, one-half of the Adam would be represented in the child. Now, if some of these organic molecules were to remain latent in the body of the offspring, and pass unchanged into the offspring of the next generation, a smaller portion of the Adam would be repeated in the grandchild. We are to suppose that each plastidule carries so much of the parent, potentially, into the child. At each successive generation less and less of the Adamic plastidules would appear, and less and less of the Adam. We should have a fractional series with unity for numerator, and an ever-increasing number for denominator. At last we should reach a term whose denominator would be infinitely large. It would express the complete elimination of the Adamic plastidules. Now, so long as any plastidules of an ancestor of any degree of remoteness remain, so long will the man or the animal inherit something from that ancestor; so long will atavism occur. When all plastidules of such ancestor are cut off, we have abbreviated heredity.

[1] To account for the facts of heredity, Darwin has formulated a theory called Pangenesis. To account for the facts of heredity and abbreviated heredity Dr. Elsberg has proposed a theory which he calls "the Conservation of the Organic Molecule." The biologist must be allowed as much "scientific use of the imagination" as the physicist. If the one must have his atoms and molecules, the other must have his physiological units, his plastic molecules, his "plastidules." Let us imagine the first pair of any race, say the human race. A child of the Adam and Eve would be derived wholly from its parents, and, if the plastidules which passed into the embryo were derived equally from each parent, one-half of the Adam would be represented in the child. Now, if some of these organic molecules were to remain latent in the body of the offspring, and pass unchanged into the offspring of the next generation, a smaller portion of the Adam would be repeated in the grandchild. We are to suppose that each plastidule carries so much of the parent, potentially, into the child. At each successive generation less and less of the Adamic plastidules would appear, and less and less of the Adam. We should have a fractional series with unity for numerator, and an ever-increasing number for denominator. At last we should reach a term whose denominator would be infinitely large. It would express the complete elimination of the Adamic plastidules. Now, so long as any plastidules of an ancestor of any degree of remoteness remain, so long will the man or the animal inherit something from that ancestor; so long will atavism occur. When all plastidules of such ancestor are cut off, we have abbreviated heredity.

[1]