ABSTRACT
Fossils are exclusively found in the so-called sedimentary deposits which form the upper part of the earth’s crust. Sediments are such rocks, nearly always appearing as layers, which by deposit of gravel, sand, or mud, are formed in the greater water-basins. ‘Specialization’ signifies, therefore, the development in one direction of an organ or of the entire organism. What causes are effective are not always demonstrable; probably the necessity of purposeful adaptation to changed environments was the reason. As an external impulse to variation Handlirsch cites the Permian glacial epoch which, particularly, as a result of the general cooling, may have caused the transition of the larval forms into perfectly metamorphosed ones as ‘being generally better fitted for cold seasons.’ The almost sudden disappearance of many multifarious groups has long presented a particularly attractive problem. The Cambrian or pre-Cambrian formations show in a general way organisms for the first time; they are sharply defined according to depth.
