GIFT OF THE LAW OF IRREVERSIBLE EVOLUTION BY BRANISLAV PETRONIEVICS, PH. D. FROM THE SMITHSONIAN REPORT FOR 1918, PAGES 42&-440 (PUBLICATION 2565) WASHINGTON GOVERNMENT TRINTING OFFICE 1920 ON THE LAW OF IRREVERSIBLE EVOLUTION BY BRANISLAV PETRONIEVICS, PH. 1). FROM THE SMITHSONIAN REPORT FOR 1918, PAGES 429-440 ( PUBLICATION 2565) WASHINGTON GOVERNMENT PRINTING OFFICE 1920 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 !')() 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 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 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. IRREVERSIBLE EVOLUTION PKTROS'tP^ TC'S. (b) By the regressive evolution of the new parts, the total disap- pearance of these parts being impossible. (c) By the ascending evolution of these new parts in a ne\v direc- tion. 3. If the original structure of an organ has been lost through de- scending evolution (with or without the loss of parts), this original structure can not be regained : (i) By the reacquisition and progressive evolution of the lost- parts, this reacquisition being impossible according to the first la\v. (ii) By the ascending evolution of the nonreduced parts in a new direction. (iii) By the ascending evolution of altogether new parts. II. The various cases falling under these three laws we wish now to explain by examples found in the writings of Dollo. For the first law examples are very numerous. The birds lost their teeth during the Cretaceous period; no subsequent bird has been able to regain these lost parts. The mandible of mammals consists of a single piece homologous with the dentary part of its reptilian ancestors; no mammal has been able to regain the lost other part., of the reptilian jaw, etc. But the examples that especially demonstrate the validity of the first law are those in which the return to ancestral conditions would necessitate the reappearance of parts which an organism has lost. As these examples are at the same time illustrations of the two other laws we shall deal with them in connection with these laws. The best-known example of the first alternative under the second law is the pseudo dentition of Odontopteryx, an Eocene fossil bird. Instead of the true teeth that have been lost, Odontopteryx has the margin of the beak and of the lower mandible dentate like a saw. The most striking example of the second alternative under the second law is the pelvis of Triceratops. The dinosaurian ancestors of Triceratops had become adapted to bipedal life, and therefore were possessed of a very long and very narrow ischium and of a pubis provided with a postpubis which was similarly very 'long and very narrow (Dollo. 10, p. 4-1-1). In its secondary adaptation to quadrupedal life Triceratops was not able morphologically to regain the triardiate pelvis of its far-distant quadrupedal ancestors, for it has retained traces of the bipedal phase in the rudimentary posi- pubis and in the narrow, recurved ischium. That is to say, the post- pubis. the new structure acquired during bipodal life, could not totally disappear, and the new form of the ischium could not dis- appear either (Dollo, 10, p. 440). The most important and most obvious example of the third alterna- tive under the second law is also found in a dinosaur, nearlv related 4-'}^ ANN'I-.\I, ;;:-.rifosaurus. This animal had for its immediate aii'.TStors bipeds like "he ance.-tors of Trieeralops. and, like Tricera- lops. it became readapted to ouadrupedal life, nut \vhile the tri- radiate pelvis of its far-away quadrupedal ancestors lias been physio- logically reestablidied ihroiiirli regressive evolution (atrophy) of poMpubis and ischium in Triceratops ( Dollo. 1. c. p. !-!-<>) these {)art> have evolved iii ;i IK : \V direction in SLeir(-:anrii . Here the i- chili in i)i'co!iir- * in'] II-I^IM! :, ii; S Hat leneil : tlie ]K>stpiib; ihe same and moreoxer applies Ux-ir eiosely alonir tlie ventral in ii\a'in o[' the ischiuin. l>ui moi'piioloii'ically tliere i- here no retn.rn to the former triradiatc cinidition of the [)elvis. since th' i>chinm has kept some I race of tlie form which it acquired in the biped pha-e. and liie |)O terior I.' rand) - !' ! iu- j <-! \ i- i:- no Ioniser formed bv the iseh mm alone hut b\' thi- eoinple.x. A\'hi!e ex-oh'inL! 1 'u: tlie new diieciion tin I hus changed in function (Dolio. !. c. p. i IT ). We find an i'lnsl! :it;on of llu v firs! alternative of (lie thinl law in the evolution of the arm?: in the Octopods. 'These anima is in adaj)t- inii 1 tliem>elve.- to i a pair of arm.- (t lie tentac- ular arm-) ; iiale ancestors (he heteropod de- capods. M. ; L'oine i>opod> autam (exc( : [)(rni!.* I lie pecu- liar cas< ctocotylisation ! ilagns. an arboreal IvMn.ii'ai'oo. The si r net u re of the fool in the ial Macropodidae- the predominance of the fourth to second and third, the reduction of ihe lil'i', li ipearance of the ii'reat toe slio\\"s us thai \\i.M'e arboreal. In Dendrolainis, a Maci'opni arboreal, llie opposable ti'roat toe. co!,; : diaie ancestors t!ic terrestrial kaiiii'ai I !.!. i>iit the unreduced |)arts of the I' l'i n^' evolution ni :< new direction. \\'iii!" i he in. i ii : iminished in length, ihe- ph;i e ieniilhened and tiie chnvs !ia\e ;! 1 the same time ! Tim- the font of Dendroiairus has tioi bei r i rucl me of the foot ol il> distant icci :i 11 v flu" I litmtfifirnini* afriratm*. in I'I 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 immediate ancestors of Dermochelys reac- quired a carapace and a plastron, but this carapace and plastron are entirely new structures of superficial dermal origin. Eeadapting 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.) ITT. 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 ethological 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 num- i See A. S. Woodward, Catalogue of the Fossil Fishes iu 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 jSature, vol. 81, 1909, p. 292 (and Dollo, 1(J, reni. 2, p. 387). 434 ANNUAL REPORT SMITHSONIAN INSTITUTION, 11)18. ber of terms intermediate between the extremes (Dollo, 5, p. 97 1 ). Just such a series we possess in the paleontological series of the Dipnoi: Dipterus valenciennesi, Dipterus macropterais, Scaumen- acia, Phaneroplenron, Uronemus, Ctenodus, Ceratodus, Protopterus, Lepidosiren (1. c., p. 88). Dollo shows that the structure of the tail as well as that of the top of the head, the squamation, the pugu- lar plates, the opercular apparatus, the ganoin, and the ossification of the mandible, the suborbital band all this proves that the course of evolution has been in the direction from Dipterus to Ceratodus. and not the opposite (1. c., p. 89-97). It is especially by the struc- ture of the tail that the concept of irreversibility is illustrated. In a long and thorough treatment of the subject Dollo shows (1. c., pp. .89-97) that the diphy cereal tail of the Dipnoi (and of the other known ancient and modern fishes) is a secondary diphy cereal tail whose morphological value in the Dipnoi (the second dorsal fin, the second anal fin) is not equivalent to the morphological value of the primitive diphycercal tail (caudal fin). In this secondary diplry- cercal condition there is therefore no return to the primitive structure. The most important other cases of phylogeny which Dollo has considered are the phylogeny of the sirenians (Dollo 3, p. 119), the phylogeny of the Leather-backed turtle (Dollo, 7, p. 9), and the phylogeny of the Holocephali (Dollo, 13, p. 107). One of the most important cases with regard to the ethological application of the law of irreversibility is found in the memoir on the Dipnoi. If it be assumed that Dipterus comes from Ceratodus, as the latter is an adaptation to life in turbid water, it would be necessary to suppose either that Dipterus represents an adaptation to life in mud (excessively turbid water), or else that it represents a return to life in clear water. The first alternative being that of Lepidosiren, the second is the only one which remains open for dis- cussion (Dollo, 5, p. 99). But, putting aside paleontological and purely ethological reasons, the law of irreversibility is sharply op- posed to such a view. "Would the lost ganoin return? Would the cephalic shield resolve itself into its ancestral elements? Would the suborbital band with its ossicles in varying number become once more a solid arch? Would the opercular appa- ratus resume its original dimensions? Would the vanished jugular plates re- appear?" As all of these structures are reduced in Ceratodus (1. c., p. 100), Dipterus can only represent a primary adaptation to life in clear water, that is to say it is purely a fish (" the most pisciform of Dipnoi," 1. c., p. 101). 1 Discussing the subject of the phylogeny of the Iloloeephali (Dollo. 13, pp. 107-108), Dollo says: " The idea of the irreversibility of evolution, which has led me to the results that. I have just demonstrated, has once more shown its usefulness. Without it one would be led to assert that specialized organisms could become primitive again and then once more specialize themselves in the same or another direction. Such an assumption, unless supported by absolutely complete 1 paleontological series which we are far from possess- ing would destroy all possibility of discovering phylogeny, the main object of nior- puology." IRREVERSIBLE EVOLUTION PETRONIEVICS. 435 Another important instance of the ethological application is fur- nished by the bipedal habits of the immediate ancestors of Stego- saurus and Triceratops. If evolution were reversible these two dinosaurs would have exactly regained their former quadrupedal structure, and there would have been no way to dis- tinguish their secondary quadrupedal existence from the first (Dollo, 10, p. 448). The other most important cases of ethological application are : The secondary adaptation to the swimming sea life of the 1 Pycnodonts (Dollo, 17, pp. 108-9), the secondary adaptation to the swimming sea life of the Trilobites Dephon and Aeglina (Dollo 16, pp. 410 and 412), etc. Among the instances of the morphological application of the law, that of the secondary abdominal ventral fins in the teleosts has a spe- cial importance. As is known, the ventrals of teleosts may be either abdominal or thoracic or jugular. But among the abdominal ventrals we have two types those which have no connection whatever with the pectoral girdle, and those joined to the clavicular symphysis by a ligament. As there is no reason for the presence of this ligament in situ we have to conclude that it is the degenerate remnant of a former direct connection with the pectoral girdle. In conformity with the irreversibility of evolution the ventrals in again becoming abdominal have kept the connection with the clavicular symphysis which they acquired when occupying a thoracic or jugular position (Dollo, 14, p. 139). The other important instances of the morphological application of the law are: (1) The very anteriorly placed choanae of the sea turtles (Dollo, 8, pp. 817-820), (2) the longirostral and brevirostral condition in Crocodilians (Dollo, 12, p. 85), etc. Finally, we must mention the one instance in which Dollo has used his law in systematic work the Ptyctodonts. Before Dollo these fossil fishes, then known from their dental plates only, had been placed among the Holocephali. In his important memoir on this subject (Dollo, 13) Dollo showed that, by virtue of the law of ir- reversible evolution, the Ptyctodonts can not be regarded as Holo- cephali and that they ought to be treated as Arthroderes. Since then Dollo's conclusion has been wholly confirmed. Although the empiric evidence for the validity of his law has been abundant and varied, Dollo was not satisfied with such a wholly empiric demonstration. He has attempted to give a deductive dem- onstration as well. He says : The Irreversibility of Evolution is not, as many have believed it to be, merely an empiric law based purely on facts of observation. But it has deep- seated causes which carry it in final analysis to a question of probabilities, us in the case of the other laws of nature. Evolution being a summation of exactly determined individual variations in an exactly determined order, to have it reversible would be to admit the possibilty of the intervention of causes exactly the inverse of those which produced and fixed the individual variations from which the first transformation arose, and in an exactly inverse 430 AXXUAL RKPORT SM 1TI 1SOX ! AX 1 XSTITI.'TTOX, 1918. order circumstances too complex for it to bo imagined that they are ever realized. (Polio, 19, p. -~9, rein.; sei> also Polio, 3, p. 127.) And, when speaking of tlie impossibility of the descent of Dip- terns from Ceratodns (Dollo, 5, p. 100, the passage referred to above) he says : And let it bo noted that it is here a question not of one isolated character, bin 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 aflirm will) 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. TV. 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 Hist 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- -ible. ft 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 -oe 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 (hi- tendency, bearing in mind, on the one hand, the tin- similar oJ^.-rvalions of A, Hamilirsrh, 1M, ].. i:^s (cilnl by lollo, lu 4 'JIM, IRREVERSIBLE EVOLUTION PETRONIEVICS. 437 difficulty of producing new tendencies in the germ by the influence of external onuses, and on the other the degree of correlation that would be needed among these tendencies. When it is a question of the reduction of an organ or part, two alternatives must be distin- guished. If the reduction has gone so far that the corresponding tendency in the germ is verging 011 complete disappearance, the re- duced organ or part will find itself practically in the same condition as if it were already lost. But if their reduction has not reached to such an extreme their evolution in an inverse direction will not be impossible. For the second law we must distinguish between the case of a single part and that of a complex organ. The regressive evolution of a sin- gle part, if during this evolution and the preceding progressive evo- lution no change of form has taken place, could clearly lead back to the point where the progressive evolution started. And the regres- sive evolution of a single part, if the corresponding arrangement in the germ is not too much enfeebled, could evidently also be followed by a new progressive evolution. But if a change of form has taken place during the first progressive evolution, and if this change of form has been so great that a change in the corresponding arrange- ment of the germ has been necessary, then neither the regressive evo- lution following the original progressive evolution will be able to lead back to the point of departure of the latter, nor will a new pro- gressive evolution have the power to accomplish it, because to do so would necessitate the return to a disappeared condition. If. for instance, a tooth has first increased in size and afterwards dimin- ished without change of form this tooth will be able by diminution to assume the dimensions which it had at the beginning of its in- crease, and a new increase of the same kind will not be impossible (if the reduction has not gone too far). But if the increase in size has been accompanied by a radical change of form, if, for instance, a conical tooth has become laterally compressed, then a return to the conical form will not be possible either during its diminution l or during a new increase in size. In the case of a complex organ Dollo asserts that its return to the previous condition through the action of regressive evolution is im- possible on account of the " indestructibility of the past." But if a single reduced part of an organ is regarded as the supposed reason why reversibility is impossible, we are able to affirm almost with cer- tainty that in such a case the indestructibility of the past does not exist, because it would find itself in contradiction with the well es- 1 This impossibility is exactly what W. D. Matthew supposes to have happened during the evolution of the FcUtlac when he supposes that the felines come from Dinictis, a primitive saber-toothed cat (see W. P. Matthew, " The Phylogeny of the Feiidae," Bull. Amcr. Mus. Nat. ITist., vol. 2S. 1910, p. 200 s ). Scott has clearly had a glimpse of the fact that this phylogeny contradicts the law of irreversible evolution . Scott, 28, p. 540 s). 1 3GG5U 20 29 438 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1918. tablished law of the necessary regressive evolution of nonfunctional parts and organs. The reversibility of the ascending evolution of a complex organ, when it depends on a reduced part, is therefore not impossible (we can suppose, for instance, that the secondary ventrals of teleosts will return in the future to their original condition through the complete disappearance of the clavicular ligaments). In the case of a nonrecluced part, whose form has, however, changed during ascending evolution, the indestructibility of the past .again does not exist in the strict sense, the nonreduced part being able to change its form again by a new progressive evolution, although the original condition of this part, and consequently the original condition of the organ in question can not be reestab- lished. The pelvis of Triceratops may be taken as an example. The postpubis of this pelvis exists in a very rudimentary condition, and as rudimentary parts tend to disappear, the postpubis certainly would have disappeared if Triceratops had lived long enough. It is therefore only its recurved ischium, very different in form from the ischium of its distant tetrapod ancestors, which was able to pre- vent Triceratops from recovering its original pelvis. Finally, if there is an ascending evolution of nonreduced parts (pelvis of Stegosaurus) it is the change of function which saves these parts from a regressive evolution ; the indestructibility of the past does not exist here either. And it is clear that the same reasoning is also applicable under the third law to the evolution of a complex organ. To sum up : The irreversibility of the evolution of a complex or- gan depends entirely on the irreversibility of the evolution of the reduced or nonreduced individual parts which enter into its com- position, and the second and third laws are not without exceptions in this respect any more than the first; as we have seen, it is the germinal base of the first law r w r hich underlies the entire subject. As I said at the beginning, most naturalists know D olio's first law only. This is only a part of his general law, although the most im- portant and most certain part. 1 This general law, in spite of the pos- sible exceptions, has an extraordinary importance for biological philosophy and evolutionary philosophy in general. Dollo will al- 1 Besides the law of irreversible evolution Dollo has formulated (see Dollo, 4, p. 165) two other laws that of discontinuous evolution (before II. de Vries) and that of limited evolution. In his subsequent writings Dollo has only rarely touched on these two other laws (on the law of limited evolution see Dollo, 7, p. 9, Dollo. S, p. 813 and p. 820, Dollo, 9, p. 131 ; on the law of discontinuous evolution see Dollo, .">. rem. (66), p. 120; Dollo. 7, rem. (11), p. 9; and Dollo, 17, pp. 139-140). IRREVERSIBLE EVOLUTION - PETRONIEVICS. 439 ways be regarded, like Cuvier before him, as the founder of a great law of the organic world. 1 LITEKATUKE. 1. L. Dollo, Cours autographic sur 1'evolution du Squelette des Vertebras. Lemons faites a 1'Institut Solvay (University de Kruxelles) en 1891-2. 2. - - g ul - rorigine de la nugeoire caudale des Ichthyosaures. Bulletin de la societe beige de Geologic, de Palcontologie cl de llydrologie, vol. 6, 1892, Proces-verbaux, pp. 167-74. 3. - Sur la morphologic des cotes. Bulletin scientifiquc dc l