I30 GEOLOGICAL BIOLOGY. 



found in every place or condition, but in each particular kind 

 of environment particular kinds of animals are found, and 

 their living is more or less dependent upon those conditions. 



Hence we infer another general hypothesis : 



(3) That the conditions of environment do in some measure 

 determine the particular form and structure of each organism. 



The Gastropoda Illustrate the Law of the Relationship between 

 Organisms and Environment. — In order to show more particu- 

 larly how the differences of form (expressed by different 

 species, genera, and families in scientific classification) are 

 related to differences in the conditions of environment, a 

 class of the Mollusca, the Gastropoda, may be examined in 

 detail. This group of organisms is convenient for the pur- 

 pose because of the full statistics already accumulated regard- 

 ing the geographical distribution of its species. 



Meaning of the Classification of Organisms. — Without defining 

 the morphological characters indicated by the classification, 

 it is important to remember that zoological classifications are 

 fundamentally based upon morphological differences, that 

 organisms of two distinct classes present greater morphologi- 

 cal difference than those of a single class, that lesser diverg- 

 ence in form is expressed by division of the class into sub- 

 classes, and that the animals of the same order present 

 greater resemblance to each other than to those of different 

 orders. Families are again subdivisions of the orders, and 

 each family includes two or more genera, and the species of 

 each genus are alike in their general form, differing only in 

 some of the more minute details. Hence when we describe 

 the peculiarities of the distribution of genera, we are express- 

 ing the law of association between the generic form and the 

 conditions of environment indicated by the geographical dis- 

 tribution. Thus, the common sea-whelk, Buccinum unda- 

 tum (Fig. 33), represents the class Gastropoda as contrasted 

 with the Dentalium (Fig. 37), belonging to the class Scaphop- 

 oda, Hylaea, a Pteropoda or Chiton (Fig. 36), a representative 

 of the clas-s Placophora. The Gastropoda, Scaphopoda, Ptero- 

 poda, and Placophora together constitute that division of 

 Mollusca called Glossophora, being alike in the possession of 

 a more or less distinct head-portion of the body, and of a 



