430 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1918. 



spread type in the domain of organic evolution) progression pre- 

 dominates, or, to put it in another way, if the final condition reached 

 represents progress in comparison with the initial condition, we then 

 shall call such a mixed evolution " ascending evolution,"' and of 

 this process the extreme type is represented by pure progressive 

 evolution with or without the addition of new parts. But if, in 

 mixed evolution, regression predominates, or, in other words, if the 

 final condition reached is a regress in comparison with the initial 

 condition, we shall call mixed evolution of this kind " descending 

 evolution," pure regressive evolution evidently representing the ex- 

 treme type of such a process. The foot of the horse, made of a 

 single digit, which has come from a pentadactyle foot by the 

 atrophy of the lateral digits and the great increase of the median 

 digit, is the best-known example of ascending mixed evolution, while 

 the skull of the living Ceratodus, in comparison with that of Dip- 

 terus, its probable Devonian ancestor, presents an example of de- 

 scending mixed evolution. 



Taking into consideration on the one hand, the definitions which 

 we have just made, and, on the other, the examples cited beyond, 

 which Dollo brought forward in favor of his law, we ought to sep- 

 arate the cases of ascending evolution from those of descending 

 evolution, something which Dollo himself did not do. Clearly, 

 if the structure of an organ or if the parts of an organ are lost 

 through descending evolution, and if it is not possible, as is almost 

 unanimously agreed, to replace the lost structure or parts, it does not 

 at all follow — at least a priori — that a reversal of evolution would 

 not be possible in the contrary case; that is, when the structure of 

 an organ has been lost by the ascending evolution of this organ. 



We should therefore replace Dollo's single law by three different 

 laws, one of which, the first and most fundamental, will express the 

 impossibility of a reacquisition of lost parts, the second of which 

 will apply to the cases in which the original structure of an organ 

 has been lost by ascending evolution, and the third to the cases in 

 which the structure has been lost by descending evolution. 



These three laws are as follows : 



1. Organs and parts of organs reduced or lost through regressive 

 evolution can not be regained by a new progressive evolution. 1 



2. If the original structure of an organ has been lost through 

 ascending evolution (with or without the addition of new parts) 

 the original structure can not be regained : 



(a) By the reacquisition of the lost parts, this reacquisition being 

 impossible according to the first law. 



1 For the first law, see Dollo, 7, p. 5 (the lost interclavicle of Dermochelys) , Dollo, 9, 

 p. 130 (the atrophied pineal eye of riioplatecarpus), and Dollo, 16, remark 2, p. 400. 



