105G UAllOX KJIANCIS XOl'CSA ON lUiVEUSlUJ.J!; 



chamctaristic and date buck at least to Pliocene time, but that 

 ai-e all the same not yet perfectly fixed. Such characters have 

 each time to be acquired by a special stimulus in every individual. 

 When such a stimulus is lacking a revei-sal to the ancestrnl type 

 takes ])lace. Such characters are, for example, among many 

 other ones, the blindness of Frotens (16) and the siia[io of the 

 calcaneus in man (17). 



Other characters, as the development of the foramen of the 

 opercvdum, tlu-ough which, in the Urodela, the extremities are 

 pi-otruded (16), or the scrotum of man (16), aie even then deve- 

 loped to a certain degree, when inciting stimuli, as the pres^sure 

 of tlie extremities against the operculum or the descent of the 

 testicles are i»ot acting, but in such cases these characters are 

 less marked than when the stimuli are acting. 



A third group of characters is always developed in ontogenesis, 

 and even appfirently without reason. These observations show 

 that in the fixing of new characters quite different stages occvir. 



Comparing now these stages Avith the changes found in the 

 orbital region, it is evident that the reversal of the neo-orbitab 

 structure to the palajo-orbital type in permian or primitive 

 reptiles (Tortoises) is entirely analogous to the case when a not 

 yet fixed character is lost agnin. The undecided condition pi-e- 

 vailing among the Sauropterygians and the Squainata can bq 

 well compared to the changes in Weidenreich's second group, 

 and the development of the tectorbital type shows that in the 

 highly developed i-eptiles the neo-orbital type had become fixed 

 to such an extent that a revei-sal was no more possible. 



In this wny pahTontological obseivations corroborate zoological 

 research, and the interest of this case lays in that it is correlated 

 with geological time. 



The changes may be shown diagrammatically as follows : — 



FalcBO-orbital tj/pe. . Neo-orbital tj/^w. Tectorbital tj/jic^ 



Vriinitive Stegoceiihaliiuis 7 \ 



,, reptiles ) \ C Speciiilised Stcgocopliiiliaus, 



^ (. pnyiirassic veptilcs 



Specialised prejuvassic ") . . ▼ 



3 ii^ ■ . Specialised ,,«-.!.- ^ v , -. 



jnrassic reptiles 3 ( Jurassic reptiles. 



reptiles ) kT . Specialised post- \ . C Specialised post- 



(The explanation of the abbreviated terms '• prejurassic " and 

 postjurassic" is given in the text.) 



Conclusion. 



After having discussed five cases of reversible evolution, four 

 of which are beyond question, and after having mentioned at the 

 beginning of the paper several cases of irreversible evolution, 

 conclusions may now be drawn. 



The fii'st certain case of reversible evolution shows how an 

 ossification, which was interrupted during the course of evolution, 



