442 The Evolution of Fishes 



have come down to us from rocks older than the end of the 

 Pliocene. The general modern type of the fish -faunas being 

 determined in the latter Eocene and the Miocene, the changes 

 which bring us to recent times have largely concerned the 

 abundance and variety of the individual species. From geo- 

 logical distribution we have arising the varied problems of 

 geographical distribution and the still more complex conditions 

 on which depend the extinction of species and of types. 



Factors of Extinction. These factors of extinction have 

 been recently formulated as follows by Professor Herbert Osborn, 

 He considers the process of extinction as of five different types: 



" (i) That extinction which comes from modification or 

 progressive evolution, a relegation to the past as a result of 

 the transmutation into more advanced forms. (2) Extinction 

 from changes of physical environment which outrun the powers 

 of adaptation. (3) The extinction which results from com- 

 petition. (4) The extinction which results from extreme spe- 

 cialization and limitation to special conditions the loss of which 

 means extinction. (5) Extinction as a, result of exhaustion." 



Fossilization of a Fish. When a fish dies he leaves no friends. 

 His body is at once attacked by hundreds of creatures ranging 

 from the one-celled protozoa and bacteria to individuals of his 

 own species. His flesh is devoured, his bones are scattered, 

 the gelatinous substance in them decays, and the phosphate of 

 lime is in time dissolved in the water. For this reason few fishes 

 of the millions which die each year leave any trace for future 

 preservation. At the most a few teeth, a fin-spine, or a bone 

 buried in the clay might remain intact or in such condition as to 

 be recognized. 



But now and then it happens that a dead fish may fall in 

 more fortunate conditions. On a sea bottom of fine clay the 

 bones, or even the whole body, may be buried in such a way as 

 to be sealed up and protected from total decomposition. The 

 flesh will usually disappear and leave no mark or at the most a 

 mere cast of its surface. But the hard parts, even the muscles 

 may persist, and now and then they do persist, the salts of lime 

 unchanged or else silicified or subjected to some other form of 

 chemical substitution. Only the scales, the teeth, the bones, the 

 spines, and the fin-rays can be preserved in the rocks of sea or 



