190 THE POPULAR SCIENCE MONTHLY. 



canal develops isolately, a section here and another there. Now, a 

 stonaach is simply an expanded portion of the canal. Let the tract 

 of the canal he laid in isolated openings, let these openings he elon- 

 gated, 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 embry- 

 otic 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 papilla? as possible. Now, these papillae, for a long 

 time mistaken for lungs, for a long time, perhaj^s, were lungs. AV e 

 have found that in Doris the gills are connected only with the diges- 

 tive system, and we may suppose that in some ancestral form of Eolis 

 pajjilliform gills were connected with this system in the same way, 

 that is, through the liver. Only a slight departure from the normal 

 development Avould 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 stom- 

 ach, 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 sufter 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 de- 

 scending 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 ten- 

 dency in every thing to become something else and better. It is de- 

 termined by many factors, inner and outer, and, as Spencer has shown, 

 *' the cooperation of inner and outer factors works changes until an 

 equilibrium is reached between tlie 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 spe- 

 cies have remained constant since the Cretaceous epoch. On the sur- 

 face of the sea and on the beach, the conditions of life have not been 

 constant, and surface and littoral species have been moi-e 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 aerial fauna must oppose to these outer factors a 

 corresponding adjustment of inner factors. The fauna of this ele- 

 ment 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 tlie uplands of Colorado. More- 



