ON THE LAW OF IRREVERSIBLE EVOLUTION. 



BY BllANISLAV ]'KTKO:\IEV-LCS, I'll. D. 



Louis Dollo, the great Belgian paleontologist, first publicly for- 

 mulated in 1893 (Dollo, 4) his famous law of irreversible evolution, 

 one of the most important laws of the evolution of organized beings. 2 

 This law has often been debated and applied, but I do not know 

 that anyone has attempted to set it forth, basing his exposition on 

 Dollo's own works. This is what 1 propose to do here, adding to 

 my account some critical remarks on the value of the law in ques- 

 tion. 



The law of irreversible evolution was stated by Dollo as follows: 



An organism can not return even in part to a previous condition already 

 passed through [deja realize] in the series of its ancestors. (Dollo. 4 p. 165. ) 3 



It is usually supposed that the law thus expressed applies only 

 to parts and organs which are reduced or eliminated; but this is 

 not correct. The law is much wider in its application, covering 

 functional organs as well. In order to understand more clearly the 

 far-reaching nature of Dollo's law we must make certain distinc- 

 tions in the concept of organic evolution and give some definitions 

 of them. 



Organic evolution may be, as we know, progressive, regressive, 

 and mixed. 1 If, during mixed evolution (which is the most wide- 



1 Translated, with permission, by Gerrit S. Miller, ir., from Science Progress January, 

 3019. 



2 He previously stated this law in 1892, in his Cours autographic, etc. (Dollo, 1), the 

 same year in a note which appeared in the Bulletin do la soc. beige do Gool., etc. (Dollo, 

 2) and in an article which appeared in A. Giard's Bulletin scientifique do la France et 

 de la Belgique (Dollo, 3). 



3 Later, Dollo expressed his law with greater exactitude : 



"An organism never exactly renews a previous condition, eveu if it finds itself placed 

 in an environment identical with one through which it has passed. But, by virtue of the 

 indestructibility of the past, it always retains some trace of the intermediate stages which 

 it has traversed." Dollo 17, p. 107, and 10. p. 443.) Let us note, that Dollo "definitely 

 admits the reversibility of conditions of existence : ; ' Evolution is irreversible as regards 

 the structure of organisms * * *, but reversible as regards environment. (Ethology) " 

 (Dollo, 7, p. 15.) 



1 Tn my course of independent lectures (on universal evolution) given at: the Sorbonne 

 this year, which will later be published, I have defined evolution in general as follows: 

 "Evolution is a thing's: coming into being by successive stages of change." When each 

 successive stage of the evolutionary process contains something more than the preceding 

 stage, evolution is progressive; it is regressive when each successive stago Contains some- 

 thing less than the preceding. Evolution is mixed when in an evolving whole one n;u-1 

 evolves progressively and the other regressively. 



429 



