SEDIMENTATION, ENVIRONMENT, AND EVOLUTION. 23 



example, the Permian inland seas, now represented by the 

 dolomitic limestones and gypsum beds of N.E. England, show 

 a fauna which bears affinities with that of the preceding period, 

 the Carboniferous, but as a whole becomes impoverished in 

 species and rarer in individuals. At the same time, distorted 

 and dwarfed forms slowly appear — apparently as a result of 

 the concentration of sea-salts which were eventually thrown 

 out of solution upon the lake bottom as a result of evaporation. 



In another example, the isolation of the great lakes, 

 such as Tanganyika, and others, in Africa, resulted in gradually 

 shrinking lakes throughout the Miocene period to the present 

 day. The institution, however, of partial internal drainage 

 systems into the lakes, together with tropical floods (in contra- 

 distinction to the aridity of the Permian), has led to the gradual 

 sweetening of the waters. The fresh-water genera of mollusca 

 with marine affinities, such as Paramelania and others, are 

 typical of the peculiar fauna of the lake, a fauna which, 

 although of Mid-Tertiary age, contains forms bearing resem- 

 blances to certain Jurassic fossils. 



In other instances we are able to note the dying out or 

 migration of species with changing climate. The gradual 

 oncoming of the Glacial Period in Western Europe was heralded 

 by the southward migration of a flood of northern mollusca, 

 such as Cardium gramlandicum, Neptunea antiqua, Natica 

 clansa, N. helicoides, Pecten (Chlamys) islandicus, Tellina 

 prcetenuis, and others, into the area o f northern Essex, and the 

 consequent movement of the warmth-loving mollusca south- 

 ward out of the British area. 



Evolution. — Many of the organisms that occur as fossils 

 can be grouped into series showing gradual transition from 

 species to species and even genus to genus. Indeed, it has 

 with truth been asserted that wherever the geological strata 

 yield a complete record, transition or continuous development 

 can be traced. Such transitions can be ascribed only to the 



