AND IRREVEBSIBLE KVOLlITION. 1057 



sets in again. Tlio second case shows the persistence of a 

 primitive stage of development in later more specialised forms, 

 and shows the subsequent development of another stage of 

 evolution through which the nncestral forms had passed long 

 ago. The third case, the mixed one, shows a similar change 

 coupled with "irreversible" evolution, and finally in the fourth case 

 three phases can be discerned — one pliase, in which a character 

 is not yet fixed, so that a reversal is possible, a second undeter- 

 mined phase, and a third, in which a reversal is impossible. In 

 the latter case a particular function can only be attained by the 

 development of a new oi-gnn. 



Reviewing the "irreversible" cases, it can easily be detected 

 that the apparent " irreversibility" is always due to the fact, that 

 either an adjacent organ or an organ having a similar function 

 is called upon to replace a degenerating organ or that in the 

 absence of such parts a new organ is developed. 



From the combination of these observations the following 

 statements can be deduced : — 



(1) An apparent irreversibility will occur Avhen a certain 

 character is already so strongly fixed that it cannot be altered. 

 Such a fixation will occur all the sooner if the dismissed organ 

 acqm'red a new function. 



(2) An apparent irreversibility Avill occur wlien some function 

 is not perfectly concentrated in a special organ, so that similarlv 

 functioning organs are ready at hand to replace each other. 



(3) An apparent irreversibility will occur when an adjacent 

 organ is ready to replace the more or less degenerated one. As an 

 example of this sort the pelvis of the Crocodiles may be men- 

 tioned, for in this case the posterior ventral ribs assumed the 

 function of the degenerated pubis and became the prepubes (new 

 hypothesis of the author). 



(4) Evolution will appear irreversible when in some organ the 

 possibility of development still exists— z. e., if the organ is yet in 

 a primitive unspecialised state. In such a case this part is l•ead^• 

 to develop new features that can replace another degenerating 

 organ. An example of this case is afforded by the development 

 of the secondary dermal armour in Dermochehjs. As Schmidt's 

 investigations (12) showed, tlie skin of the Tortoises is not yet 

 strongly modified in the young, and so it has evidently not yet 

 lost the general faculty of developing dermal ossifications. 



When one of the four enumerated ways of solving a bio- 

 logical problem has become impossible, an animal can only be 

 saved from extinction by a reversal to an embryonic stage. 

 This will only be possible — 



^ (1) if the state to ho given up is not yet fixed by heredity. 

 That means if this state is not very far back in the history. A 

 good example is afforded by the development of the supraorbital 

 region ; 



(2) if the embryonic state to be called upon has not in tlu^ 

 meantime acquired a new and vital function. For this case 



