138 GEOi^OGICAL BIOLOGY. 



branch fauna species of the genera Bela, Eglesia, Fossarus, 

 Mangelia, Murex, Odostomia, Pleurotoma, Rissoa, Triforis, 

 and Turritella. 



(5) The Abysmal Zone. — 500 metres, or 100 fathoms, or 

 more in depth, down to the profound depths, supports species 

 of the genera AcHs, Acirsa, Cerithium, Chenopus, Defranchi, 

 EuHma, Fusus, Hela, Natica, Odostomia, Pleurotoma, 

 Rissoa, Taranis, and Trophon. 



Evidence of Adjustment of the Morphological Character to the 

 Environment. — An examination in the like manner of the dis- 

 tribution of species shows an adaptation of each species to 

 much more restricted bathymetric conditions, and to restricted 

 geographical areas or provinces. This fact might, however, 

 be accounted for by migration and sorting out of species from 

 choice, or the selection of environmental conditions; but in 

 the case before us, where not only genera, but whole families, 

 — families whose representatives are found in all parts of the 

 globe, — are restricted to special conditions of environment, it 

 seems impossible to account for the fact except by the sup- 

 position that the morphological characters of the organisms 

 are adjusted to the environment. 



When we examine animals whose structure is more 

 strongly contrasted, as in the case of the fish swimming in 

 water, the beast walking on land, and the bird flying in the 

 air, we are not impressed so much by the morphological 

 adjustment as by the physiological necessity of the restriction 

 to a particular environment ; but in the case of the Gastropods, 

 where the differences in form are relatively of small physio- 

 logical significance, the finding of a close correlation existing 

 between the specific, generic, and even family form, and the 

 particular conditions of environment seen in the zones of the 

 ocean, and climatal differences of land, impresses one vividly 

 with tJic immediate eonneetion betiveen differences of form and 

 ■differences of oivironment. 



Law of the Adjustment of Organisms to Conditions of Environ- 

 ment. — We learn from these statistics that the morphologi- 

 cal differences, which are the basis of the classification of the 

 various species of the ctenobranch Gastropods into genera 

 and families, are intimately connected with the differences in 



