PROGRESSION AND RETROGRESSION. 189 



rich to be limited to one or two efficient causes carried it to tlie 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 wliat 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 tlie axis of the body. The reproductive system is still 

 more askance. It is altogether one-sided. Very suggestive facts. 

 We find one-sided growth witliout the conditions whicli induce it. 

 These conditions must Jiave i)ertained 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 environ- 

 ment 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 bL^come straight. Anotlier suggestive fact. AYe find in 

 these naked mollusks heredity and abbreviated heredity.' Aplysia 

 and Doris inherit the ancestral twist. In Eolis the heritage is cut off. 



From symmetry to asymmetry, from a bivalve to a univalve, Na- 

 ture has moved, closing a cycle of evolution in the snail ; from asym- 

 metry 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 wc may find the rationale 

 of Aj)lysia's many stomachs, and Eolis's branching stomach and he- 

 patic cells. In the snail, perhaps in all Gasteropods, the alimentary 



' To accuuut for the facts of heredity, Darwiu has formulated a theory called Pan- 

 ffenexis. To account for the facts of heredity and abbreviated heredity Dr. Elsberg has 

 proposed a theory which he calls " the Conservation of the Organic Molectile." 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 physio- 

 logical units, his plastic molecules, his ^^ plastichiles." Let us imagine the first pair of 

 any race, say the human race. A child of the Adam and Eve would be derived wholly 

 Irom 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 ths grandchild. We are to supp6se 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 oif, we have abbreviated heredity. 



