GEOGRAPHICAL DISTRIBUTION. I43 



conditions of environment, within the same group, be it 

 order, suborder, family, or genus, there are also those having 

 the same structure which are not so closely adjusted to the 

 environmental conditions. 



Species Generally Closely Adjusted to Particular Conditions. — 

 This is the case until we reach the species. Species do appear 

 to be closely adjusted to some particular set of physical con- 

 ditions. Each one is so constructed that one environment is 

 at least most favorable, and to remove it from such condition 

 is either impossible without killing it, or leads to some adjust- 

 ment of its habits, and, it may be, structure and form to 

 adapt it to the changes. The adaptation can only be varietal 

 for a single individual ; hence it is only among the specific 

 characters that we find the evidence of immediate change of 

 form to adapt the organism to changed conditions. 



Fresh-water Families ; Restriction in their Distribution. — The 

 following families are made up of fresh-water species : Palu- 

 dinidjE, Ampullarida^, Valvatidae, Melaniidae, and Hydrobiidae ; 

 the latter two families containing a few brackish-water species. 



Such species are by their specialized structure restricted, 

 therefore, to continental or island habitat. 



The Paludinidae and the Valvatidae are restricted to the 

 northern hemisphere, are mainly in temperate zones, and 

 are not known south of the equator. 



The Ampullaridae are from Central and South America, 

 Eastern Africa, Madagascar, S. Asia, Malaysia, the Philippines, 

 Australia, and vicinitj, . 



The MelaniidcX are chiefly intertropical species, being 

 most abundant in India, Indo-China, Malaysia, the Philip- 

 pines, Oceanica, Africa, Central America, South America, 

 running from Central America up into Mexico, and from 

 North Africa to Spain and Asia Minor. 



The Hydrobiidae, which, according to Fischer, have been 

 distributed under eighty genera, are scattered over almost all 

 the lands between the temperate zones of the northern and 

 southern hemispheres. 



Two Closely Allied Families, Separated in their Distribution. — 

 The Strombida^ and Chenopodida^ illustrate this law. The 

 shells of both these families are heavy, and more or less 



