140 GEOLOGICAL BIOLOGY. 



of form. What we have been illustrating is the fact that the 

 species of a genus, or the genera of a family, are found adapted 

 to different kinds of environment, and that the adaptation is 

 expressed by modification of the form of the organism. In 

 geographical distribution proper the fact is emphasized that 

 likeness of characters of form is associated with continuity of 

 like environmental conditions; viz., that the same variety is 

 restricted to a particular geographical area or province. 



Geographical distribution emphasizes the fact that en- 

 vironment, by the law of adaptation, has the effect of confin- 

 ing the descendants of common parents within boundaries, 

 and thus tends to the continuance of like characters. Bathym- 

 etric distribution emphasizes the fact of adaptation itself, 

 by showing that the morphological differences distinguishing 

 the several species of a common genus, or the several genera 

 of a common order, are directly associated with differences in 

 the environment. The two groups of facts together point to 

 a most important biological law : that divergence of morpho- 

 logical characters is in some way associated with changing of 

 environmental conditions. 



Distribution implies migration, and when we observe that 

 migration is accompanied with modification and adjustment 

 to new environment, we discover this second of the funda- 

 mental laws of evolution. 



Relation between Zonal Adaptation and Geographical Range. 

 An analysis of the classification of the Mollusca shows that 

 the Gastropod structure is adapted to all kinds of environ- 

 ment, because we find genera of Gastropoda in each of the 

 several zones expressing the full range of environmental dif- 

 ferences on the earth, from the abysses of the sea to the top 

 of the dry land. 



Three of the orders of Gastropoda are somewhat special- 

 ized in adaptation to environment : the order Heteropoda 

 are pelagic forms; the Opisthobranchia are all marine, liv- 

 ing in the zones from Littoral down to the Nullipore zone. 

 The Pulmonata are restricted in adjustment to the high 

 littoral only of the marine zones, and to brackish, fresh-water, 

 and land conditions above the tide-level. The order Proso- 

 branchia has genera in every zone distinguished in our list. 



