!')() AXXl'AL RKroiiT S.MITMSOXIAX 1 "XSTITI'TK *X, IM^. 



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

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

 rcpre.-enl.- progress in comparison with the initial condition, \ve then 

 hal! ca.il such, a mixed evolution "ascending evolution, and of 

 I hi:- process the extreme type is represented b\ pure progressive 

 evolution with or without the addition of nev> parts. !>ut it, in 

 'nixed evolution, regret-ion predominates, or, in oilier words, if the 

 I'ma! condition reached i^ a repress in comparison with the initial 

 condition, \ve i-hall call mixed evolution of this kind "descending 

 evolution." pure regressive evolution evidently representing ihe ex- 

 nvme type oi : Hich a process. Tlie foot ci the In- ide <>f a 



.--in. 'lie di:.''it. v\'h.ich bar come from a [)entadactyle loot, hy the 

 atroph\ of tiie lateral di<iii> and the o-i'eal increase of the median 

 dipl. i- the best-known example of ascending' mixed evolution, while 

 iht> skull of ihe living Ccratodii", in coni[)arison with that of Dip- 

 leru>. H- [)i:obahle Devonian ancestor. [)re-ent,s an example of de- 

 seen din?/ mixed evolution. 



Taking into consideration on (he one hand, the definitions which 

 we have just made, and, on the other, the example- cited beyond,. 

 which Doilo brought forward in favor of his law, we ou.idit to sep- 

 arate the cases of ascending evc^ntion from those of descending 

 evolution, .-on set him;' which Dollo himself did nor do. Clearly, 

 if the structure of an orii'jui or if the parts of an organ are lost 

 through <//*< evolution, and if it is not possible, as is almost 



iniuioush re] >lace the lo.-t -I rues lire or part.--, it does not 



ist a priori iliat a reversal of evolution would 

 not : :'' contrary case: that is, when the structure of 



;ii- oi'i'a! <> lost hy 'lie o.HC.cndhuj CN'oiutjou of tins organ. 



refoj'e replace Dollo'- -ingle Jaw by three ditferent 

 . lir.-t and most fundamental, will express the 



y <>!' a rcac(juisit ion of inst j)aii-, the second of which 

 \\ill >ppl\ to the ease.- in which tlie original structure of an organ 

 ii-i- -cending evolution, and the third to the cases in 



\\hi has hcen lo-st by descending e\'olution. 



; i - i'o i 1 < ; vv - ; 



of organs reduced or lo.-j. through regressive 

 ca ; ' e regained o\ r a new pro^'resHve evolution. 1 

 :/. if the original structure of an organ ha- been in.-i through 

 voli or without ihe addition of new ]>art.s) 



i he ' i'co'ii mi < i : 



i ie !' >s1 |i;i rt : [ii isil i: >n being 



iUlDO:- iblc ;,rr.,rdiu'/ !o ihe first !:i\V. 



