142 GEOLOGICAL BIOLOGY. 



thus are restricted to a habitat above sea-level, are included in 

 the five families Cyclophoridae, Cyclostomidas, Aciculidae, 

 Truncatellidae, and Assimineida;. 



Of the eighteen genera of the first family, twelve are re- 

 stricted to Southern and Eastern Asia and neighboring islands ; 

 one genus (Pomatias) is distributed over North Africa and 

 South Europe ; another (Craspedopoma) over the Canary, 

 Madeira, and Azores Islands. Another genus (Megalomo- 

 stoma) is found in the Antilles and Guatemala, and Apero- 

 stoma in Central and South America and Mexico. 



The genera of Cyclostomidae have a similar distribution, 

 mainly in the lands bordering the Indian Ocean, and a couple 

 of genera (Choanopoma and Cistula) in the corresponding 

 lands bordering on the Gulf of Mexico. 



The other three families, Aciculidae, Truncatellidae, and 

 Assimineidae, are all found within the same areas. 



Families whose Genera have a very wide Range of Adaptation, 

 and Restricted Adjustment only among the Species. — If we pursue 

 the analysis still further, we find that there are some families, 

 like Cerithiidae, in which for some of its genera there is still 

 a very wide adaptation to conditions of environment ; species 

 of the genus Cerithium are living now between tide, and have 

 also been dredged from the abysmal zone. In such families 

 the zonal adaptation can be found only among the species. 



Great Difference in the Closeness of Adjustment of the Charac- 

 ters of different Taxonomic Rank. — It is hence evident that 

 there is great difference in the extent to which organisms are 

 adjusted to restricted conditions of environment. In some 

 organisms their class characters are strictly adjusted to a par- 

 ticular group of environmental conditions, as is the case with 

 the insect whose mature structure with tracheal breathing 

 restricts it to a habitat in which air is accessible ; but even 

 among insects there are cases of adaptation to life in water. 

 In other cases one order is adapted to one mode of life and 

 another to a different condition of environment — as among 

 the reptiles there are aquatic Saurians, the Enaliosauria, and 

 the true lizards, Lacertilia, adapted to live on land. In such 

 cases as we have been considering, though there are in each 

 group some cases of restriction of adjustment to particular 



