436 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1918. 



order — circumstances too complex for it to be imagined that they are ever 

 realized. ( Dollo, 19, p. 59, rem. ; see also Dollo, 3, p. 127. ) 



And, when speaking of the impossibility of the descent of Dip- 

 terus from Ceratodus (Dollo, 5, p. 100, the passage referred to 

 above) he says: 



And let it be noted that it is here a question not of one isolated character, 

 but of a whole group of characters, something that is much more serious so 

 far as irreversibility is concerned * * * But it is particularly in its 

 action on elements as multiple as these that we can affirm with certainty that 

 evolution is not reversible (1. c. rem. 72, p. 122). 



The irreversibility of evolution becomes, therefore, according to 

 Dollo, the more probable as the number of elements increases, and 

 it is practically a necessity when the number of elements is con- 

 siderable. 



IV. 



Having explained the law of irreversible evolution, the various 

 cases which it makes clear, also its applications, and its logical 

 basis, we now wish to make some critical remarks on the various 

 aspects of the law. 



In the first place, its logical basis. The deductive demonstration 

 of his law, attempted by Dollo, is very doubtful. As to the number 

 of elements on which evolution acts, it is not a question of cells, 

 but of organs and parts of organs (because it is only these last 

 which have their peculiarities determined in the germ), and the 

 number of these organs and parts is not relatively great even in the 

 most complex organism. But, if we consider the much greater 

 number of individuals in which the organs and parts of organs show 

 individual variations, the chance that they will vary in different 

 directions and consequently also in inverse directions becomes pos- 

 sible. It is only if we assert that individual variations are rela- 

 tively not very numerous — predetermined — that this course of rea- 

 soning founded on pure probability becomes weak. In that case, 

 however, the law of irreversible evolution is not the result of nu- 

 merical probability, but the result of unknown internal causes of 

 organic evolution. 



There is, therefore, no logical necessity in the law of irreversible 

 evolution, and this law remains a purely empirical rule. Let us 

 now see how much the three laws of this evolution are confirmed by 

 experience, and to what extent we should expect possible exceptions. 



As to the first law, 1 this law appears to be without exceptions so 

 far as it applies to lost organs and parts. For the loss of an organ 

 or of a part having become final by the loss of the corresponding 

 tendency in the germ, it is almost impossible to imagine the reap- 

 pearance of this tendency, bearing in mind, on the one hand, the 



1 Compare the similar observations of A. Ilandlirsch, 24, p. 132S (cited by Dollo, 15 

 rem. (2), p. 429). 



