IRREVERSIBLE EVOLUTION PKTROS'tP^ TC'S. 



(b) By the regressive evolution of the new parts, the total disap- 

 pearance of these parts being impossible. 



(c) By the ascending evolution of these new parts in a ne\v direc- 

 tion. 



3. If the original structure of an organ has been lost through de- 

 scending evolution (with or without the loss of parts), this original 

 structure can not be regained : 



(i) By the reacquisition and progressive evolution of the lost- 

 parts, this reacquisition being impossible according to the first la\v. 



(ii) By the ascending evolution of the nonreduced parts in a new 

 direction. 



(iii) By the ascending evolution of altogether new parts. 



II. 



The various cases falling under these three laws we wish now to 

 explain by examples found in the writings of Dollo. 



For the first law examples are very numerous. The birds lost their 

 teeth during the Cretaceous period; no subsequent bird has been 

 able to regain these lost parts. The mandible of mammals consists 

 of a single piece homologous with the dentary part of its reptilian 

 ancestors; no mammal has been able to regain the lost other part., 

 of the reptilian jaw, etc. 



But the examples that especially demonstrate the validity of the 

 first law are those in which the return to ancestral conditions would 

 necessitate the reappearance of parts which an organism has lost. 

 As these examples are at the same time illustrations of the two 

 other laws we shall deal with them in connection with these laws. 



The best-known example of the first alternative under the second 

 law is the pseudo dentition of Odontopteryx, an Eocene fossil bird. 

 Instead of the true teeth that have been lost, Odontopteryx has the 

 margin of the beak and of the lower mandible dentate like a saw. 



The most striking example of the second alternative under the 

 second law is the pelvis of Triceratops. The dinosaurian ancestors 

 of Triceratops had become adapted to bipedal life, and therefore 

 were possessed of a very long and very narrow ischium and of a 

 pubis provided with a postpubis which was similarly very 'long and 

 very narrow (Dollo. 10, p. 4-1-1). In its secondary adaptation to 

 quadrupedal life Triceratops was not able morphologically to regain 

 the triardiate pelvis of its far-distant quadrupedal ancestors, for it 

 has retained traces of the bipedal phase in the rudimentary posi- 

 pubis and in the narrow, recurved ischium. That is to say, the post- 

 pubis. the new structure acquired during bipodal life, could not 

 totally disappear, and the new form of the ischium could not dis- 

 appear either (Dollo, 10, p. 440). 



The most important and most obvious example of the third alterna- 

 tive under the second law is also found in a dinosaur, nearlv related 



