canal develops isolately, a section here and another there. Now, a stomach is simply an expanded portion of the canal. Let the tract of the canal be laid in isolated openings, let these openings he elongated, each, into a tube, and let the original openings be marked as pouches along this continuous tube, and we have Aplysia's row of stomachs. It is after the pattern of the digestive tube of an embryotic Gasteropod.
In Eolis the branching alimentary canal lies along the dorsal side, not the ventral. In getting itself straight, it seems to have got itself as near the dorsal papillæ as possible. Now, these papillæ, for a long time mistaken for lungs, for a long time, perhaps, were lungs. We have found that in Doris the gills are connected only with the digestive system, and we may suppose that in some ancestral form of Eolis papilliform gills were connected with this system in the same way, that is, through the liver. Only a slight departure from the normal development would transfer the connection of a gill-bud from one part of the digestive system to another, from the liver to the stomach. If, then, it would be for the advantage of the animal to have more stomach, we can see how, by Natural Selection, all the gill-buds or papillae would, in the end, cease to respire for the liver and become diverticula of the stomach. What would become of the liver? Losing its lung, it would suffer degradation. It would abort, lapse into a few hepatic cells, and become a mere vestige.
The naked Tunicates are intelligible as initial terms of a molluscan series. The naked Gasteropods are intelligible as final terms of a descending series, as impoverished heirs of an ancient house.
We have chosen for our study these slugs of the sea to develop a phase of evolution not generally understood. Evolution does not imply an unbroken course of progression. It does not imply a tendency in every thing to become something else and better. It is determined by many factors, inner and outer, and, as Spencer has shown, "the coöperation of inner and outer factors works changes until an equilibrium is reached between the organism and its environment."
On the deep-sea bottom the environing actions remain constant age after age, and we find that in the abyssal world a number of species have remained constant since the Cretaceous epoch. On the surface of the sea and on the beach, the conditions of life have not been constant, and surface and littoral species have been more subject to change. The air is more fickle than the sea. It is now warm and now cold; now moist and now dry; now in motion and now at rest: and the aërial fauna must oppose to these outer factors a corresponding adjustment of inner factors. The fauna of this element we should find the most unstable, and so we do. The only insect known to have come down to our times from times so remote as the Cretaceous, unchanged or changed but little, is the tiger-beetle of our sea and lake shores, and the uplands of Colorado. More-
