RESULTS OF PAL^ONTOLOGICAL RESEARCH 43 



precisely when they have attained the maximum of 

 prosperity, either with regard to the dimensions of 

 their bodies or in the perfection of their weapons of 

 attack and defence/ l 



In the end, however, horns of two metres in 

 breadth, as were borne by the gigantic deer, must be- 

 come directly detrimental ; reduction of a specialization 

 carried so far appears, however, to be impossible (law 

 of the irreversibility of evolution). The increase in 

 size has also its limits ; if overstepped the size leads 

 to clumsiness and unwieldiness. If the environment 

 then be altered, such forms, so peculiarly modified in 

 the one suitable direction, are doomed to extinction. 2 



Such environmental changes on a large scale certainly 

 occurred. We may consider only the various mighty 

 ice invasions which repeatedly took place : the first 

 certainly occurred already in the pre-Cambrian epoch, 3 

 another in the Permian/ and the last great one was 

 that of the Diluvium. We may further consider the 

 frequent incursions of the ocean, and the climatal 

 oscillations, which resulted in our finding in one and 

 the same region the remains of tropical or sub-tropical 



1 Deperet-Wegner : Die Umbildung der Tier welt, p. 219. 



2 Recently the palaeontologist R. Homer has exhaustively discussed 

 the question of Extinction in his work Das Austerben der Arten und 

 Gattungen sowie der grosseren Gruppen der Tier- und Pflanzenwelt, Graz, 

 1911. In essentials he agrees with Deperet's views. 



3 J. Walther: Ueber algonkische Sedimente, in Naturw. Rundschau, 1910, 

 p. 158. E. Kayser : Lehrbuch der Geolog. Formationskunde, p. 50, Supple- 

 ment. (Clear traces are found of same in N. America, Norway, and China.) 



4 E. Philippi : Ueber die Permische Eiszeit, u. Zentralblatt fur Mineralogie, 

 Geologie, und Paldantologie, 1908, p. 353. The author regards it as * proved.' 



