IRREVERSIBLE EVOLUTION PETRONIEVICS. 433 



ancestors, which possessed an opposable great toe, syndactyly of the 

 second and third toes, dominant fourth toe, and reduced claws. (See- 

 Dollo, 6, pp. 194 and 199.) 



Finally, for the third alternative of the third law we have an 

 illustration in the secondary carapace and plastron of the turtle 

 Dermochelys. The distant ancestors of this turtle were, like it, sea- 

 turtles; its reduced primary plastron (a ring formed of four pieces) 

 and its still more reduced primary carapace (represented by the 

 nuchal plate alone) bear witness to the fact. When adapting them- 

 selves to littoral life the kftmediate ancestors of Dermochelys reac- 

 quired a carapace and a plastron, but this carapace and plastron are 

 entirely new structures of superficial dermal origin. Readapting 

 itself to marine life Dermochelys has preserved this carapace and 

 plastron of its immediate ancestors, although both are already much 

 reduced. (See Dollo, 7, pp. 9-14.) 



III. 



The importance of the law of irreversible evolution is multiple. 

 In the first place, this law has a phylogenetic application, that is, it 

 places us in position to reconstruct, with the often insufficient 

 paleontological material which we possess, phylogenetic series which, 

 if they are not true series are at least series which represent indubi- 

 table evolutionary stages. Its etiological application is yet more 

 considerable. It is often the only means of determining the con- 

 ditions of existence and the method of adaptation to life of fossil 

 organisms. But sometimes this law has a morphological importance 

 also, because by using it we can distinguish homologies from mere 

 analogies. Finally Dollo has shown that it can act also as a guide 

 in classification, that it therefore has a systematic application. 



The most important phylogenetic application of the law was made 

 by Dollo in the difficult question of the phylogeny of the Dipnoi. 

 Dollo 's very ingenious paper on this subject (Dollo, 5) should be 

 considered a model presentation of the philosophic point of view 

 in the new paleontology. Before Dollo this subject was in a truly 

 chaotic state, one of the most eminent paleontologists having de- 

 clared Dipterus, the oldest and most primitive type, to be the most 

 specialized. 1 Nowhere else has the conception of the irreversibility 

 of evolution given such brilliant results. Since this conception ex- 

 presses the idea that we never fully return to an ancestral structure 

 it can be used to determine whether a particular condition is primary 

 or secondary. Consequently it can be used to decide upon the direc- 

 tion of evolution when we have a series containing a sufficient nuin- 



1 See A. S. Woodward, Catalogue of the Fossil Fishes in the British Museum, pt. 2, 

 1891, p. XX. But after Dollo published his important memoir Woodward accepted his 

 conclusions. See his presidential address to the Section of Geology, in Nature, vol. 81, 

 1909, p. 292 (and Dollo, 16, rem. 2, p. 387). 



