Newton’s System of the World and Comets

Isaac Newton’s cosmology apparently involved regular apocalypses caused by comets overstoking the furnace of the Sun and the repopulation of the Solar System by new intelligent species. He supports this speculation with an interestingly-incorrect anthropic argument.
history, philosophy, technology, insight-porn
2016-06-132019-01-11 finished certainty: highly likely importance: 2

Isaac New­ton pub­lished few of his works, and only those he con­sid­ered per­fect after long de­lays. This leaves his sys­tem the world, as de­scribed in the Prin­cipia and else­where, in­com­plete, and many ques­tions sim­ply un­ad­dressed, like the fate of the Sun or role of comets. But in 2 con­ver­sa­tions with an ad­mirer and his nephew, the el­derly New­ton sketched out the rest of his cos­mogony.

Ac­cord­ing to New­ton, the so­lar sys­tem is not sta­ble and must be ad­justed by an­gels; the Sun does not burn per­pet­u­al­ly, but comets reg­u­larly fuel the Sun; and the fi­nal re­sult is that hu­man­ity will be ex­tin­guished by a par­tic­u­larly large comet caus­ing the sun to flare up, and re­quir­ing in­tel­li­gent alien be­ings to arise on other plan­ets or their moons. He fur­ther gives an an­thropic ar­gu­ment: one rea­son we know that in­tel­li­gent races reg­u­larly go ex­tinct is that hu­man­ity it­self arose only re­cent­ly, as demon­strated by the re­cent in­no­va­tions in every field, in­con­sis­tent with any be­lief that hu­man be­ings have ex­isted for hun­dreds of thou­sands or mil­lions of years.

This is all in­ter­est­ingly wrong, par­tic­u­larly the an­thropic ar­gu­ment. That New­ton found it so ab­surd to imag­ine hu­man­ity ex­ist­ing for mil­lions of years but only re­cently un­der­go­ing ex­po­nen­tial im­prove­ments in tech­nol­ogy demon­strates how coun­ter­in­tu­itive and ex­tra­or­di­nary the In­dus­trial & Sci­en­tific Rev­o­lu­tions were.

Boldly Sir Isaac New­ton, in his , laid out the laws of ce­les­tial mo­tion—but did­n’t solve many other ques­tions about the So­lar sys­tem: how was the So­lar Sys­tem cre­ated and how old was it, how did God keep it sta­ble rather than chaotic, how long would the Sun burn un­til its fire went out, what is the fate of hu­man­i­ty, and what the role of comets?

What did he make of those prob­lems? New­ton de­clined to com­mit his be­liefs to pub­li­ca­tion, but an in­ter­est­ing sum­mary sur­vives thanks to a rel­a­tive of his.


David Gre­gory sum­ma­rizes some 1694 com­ments by New­ton, then 52, about the sta­bil­ity of the so­lar sys­tem (from pg365–371 of The Cor­re­spon­dence of Isaac New­ton (Turn­bull 1961), Vol­ume III):

[New­ton says] that a con­tin­ual mir­a­cle is needed to pre­vent the Sun and the fixed stars from rush­ing to­gether through grav­i­ty: that the great ec­cen­tric­ity in Comets in di­rec­tions both differ­ent from and con­trary to the plan­ets in­di­cates a di­vine hand: and im­plies that the Comets are des­tined for a use other than that of the plan­ets. The Satel­lites of Jupiter and Sat­urn can take the places of the Earth, Venus, Mars if they are de­stroyed, and be held in re­serve for a new Cre­ation.

In the later 1724 “Ac­count of a con­ver­sa­tion be­tween New­ton and Con­duitt”, the now-aged (83) re­veals con­sid­er­ably more de­tails of his spec­u­la­tions about the true sys­tem of the world to his nephew-in-law :

…there was a sort of rev­o­lu­tion in the heav­enly bod­ies that the vapours & light emit­ted by the sun which had their sed­i­ment as wa­ter & other mat­ter had gath­ered them­selves by de­grees into a body & at­tracted more mat­ter from the plan­ets & at last made a sec­ondary planet (viz. one of those that go round an­other plan­et) & then by gath­er­ing to them & at­tract­ing more mat­ter be­came a pri­mary plan­et, & then by in­creas­ing still be­came a comet which after cer­tain rev­o­lu­tions by com­ing nearer & nearer the sun had all its volatile parts con­densed & be­came a mat­ter fit to re­cruit & re­plen­ish the sun…as a fagot would this fire if put into it…

…that would prob­a­bly be the effect of the sooner or lat­er…per­haps have 5 or 6 rev­o­lu­tions more first, but when­ever it did it would so much in­crease the heat of the sun that this earth would be burnt & no an­i­mals in this earth could live.1

…He seemed to doubt whether there were not in­tel­li­gent be­ings su­pe­rior to us who su­per­in­tended these rev­o­lu­tions of the heav­enly bod­ies by the di­rec­tion of the supreme be­ing—

He seemed to be very clearly of opin­ion that the in­hab­i­tants of this earth were of a short date & al­leged as one rea­son for that opin­ion that all arts as let­ters long ships print­ing—nee­dle &c2 were dis­cov­ered within the mem­ory of His­tory which could not have hap­pened if the world had been eter­nal

…when I asked him how this earth could have been repeo­pled if ever it had un­der­gone the same fate it was threat­ened with here­after by the Comet of 1680, he an­swered that re­quired the power of a cre­ator—

…[I] told him I thought he owned there what wee had been talk­ing about—viz. that the Comet would drop into the sun, & that fixed stars were re­cruited & re­plen­ished by Comets when they Dropt into them, & con­se­quently the sun would be re­cruited too & asked him, why he would not own as freely what he thought of the sun as well as what he thought of the fixed stars—he said that con­cerned us more, & laugh­ing added he had said enough for peo­ple to know his mean­ing—

The System of the World

New­ton ex­plains fully “what he had often hinted to me be­fore” to Con­duitt: ce­les­tial bod­ies grow by ac­cre­tion due to grav­i­ty, and as they grow big­ger, pass from moons to plan­ets to even larger (!) comets. Comets, in loop­ing past the Sun, slowly be­come cooked. The Sun would go out due to its con­stant con­fla­gra­tion, but for­tu­nate­ly, it is con­stantly re­newed and pow­ered by the fresh fuel pro­vided it by comets pass­ing near­by. “In­tel­li­gent be­ings” (an­gel­s?) over­see this whole process of reg­u­lar fu­el­ing of the sun, but un­for­tu­nately the passed so close to the Sun that it seems likely that it will soon fall di­rectly into the sun, rather than feed­ing it a small mea­sure of fu­el.

With an enor­mous quan­tity of fuel abruptly dumped into the Sun in­stead of dis­pensed over eons, it will flare up like a bon­fire and quite likely roast the Earth (like a might, in­ci­den­tal­ly), killing every­thing on it. This pos­si­bly hap­pens reg­u­lar­ly, since hu­man­ity seems to have been cre­ated only re­cent­ly, as ev­i­denced by how re­cently such ma­jor in­no­va­tions like print­ing or nee­dles had been made (con­tra­dict­ing any sup­po­si­tion hu­man­ity had ex­isted for more than a few thou­sand years). After this, pos­si­bly God would re­new cre­ation by re­pop­u­lat­ing in­stead the moons of Sat­urn or Jupiter3 which would have es­caped the in­ferno rel­a­tively in­tact due to their dis­tance.

This is an in­ter­est­ing cos­mol­ogy and has a lot of sense to it (how does the Sun burn more than a long time with­out a mag­i­cal process like ‘fu­sion’ or else reg­u­lar re­sup­ply? pace Lord Kelv­in’s well-rea­soned & acute but wrong com­ments on the Sun’s age, comets as in­ad­e­quate fuel sources, & re­fut­ing Evo­lu­tion), but is still very alien. An­gels in charge of comets! Things re­ally were differ­ent then.

It would make for an ex­cel­lent retro SF or nov­el, if noth­ing else. (Nat­u­ral­ly, the he­roes would be re­cruited by the an­gels to help deal with the cri­sis of the re­turn of New­ton’s comet… Per­haps would like to write a fol­lowup to “Ex­ha­la­tion” or “Sev­enty Two Let­ters”, or Scott Alexan­der & ?)


It’s also in­ter­est­ing for why it’s wrong: New­ton needs comets to be at least plan­et-sized, be­cause comets are too rare to be plau­si­bly fuel sources for the Sun if they’re small (they would­n’t dump enough fuel in to keep com­bus­tion go­ing for an­other few decades/­cen­turies un­til the next big comet, as as­tro­nom­i­cal records since the Baby­lo­ni­ans con­strained big comets to be highly in­fre­quen­t), but of course they’re very small; if they were plan­et-sized, you’d think they’d se­verely dis­turb plan­e­tary or­bital cal­cu­la­tions, so was the ex­ist­ing as­tro­nom­i­cal data in­suffi­ciently pre­cise to prove the ab­sence of such or­bital dis­tur­bances or was there some other is­sue?

The an­thropic/ ar­gu­ment for the short du­ra­tion of the hu­man race is also wrong, since we know anatom­i­cally mod­ern & pre­sum­ably equally in­tel­li­gent hu­mans have been around for at least 50,000 years at this point, with hu­man civ­i­liza­tion de­vel­op­ing only in the last small frac­tion. What’s par­tic­u­larly in­ter­est­ing about his ar­gu­ment is that if he had made it at a truly ran­domly cho­sen point in hu­man his­tory (picked at ran­dom from, say, 50,000 BC to 2019 AD), then he would have cor­rectly con­cluded the op­po­site, that the hu­man race was an­cient, due to the lack of any dis­cernible progress in the re­cent past (): in fact, he could only have made this ar­gu­ment in a tiny win­dow be­tween the start of the Sci­en­tific/In­dus­trial Rev­o­lu­tion and the ar­chae­o­log­i­cal/­ge­o­log­i­cal/evo­lu­tion­ary proof of mankind’s an­tiq­uity start­ing around the 1800s, so maybe 400 years or 0.4 mil­len­nia; he had to have the bad luck to be born into that ex­act 0.8% his­tor­i­cal win­dow for the ar­gu­ment from progress to be con­vinc­ing & wrong! Re­mark­able. (New­ton was not the only one to no­tice how un­usual a pe­riod he lived in; noted some­thing sim­i­lar ~150 years prior in his 1575 au­to­bi­og­ra­phy The Book Of My Life, al­though since it is in a chap­ter sub­headed “Con­cern­ing Nat­ural Though Rare Cir­cum­stancs of My Own Life”4, he ap­pears to have bit­ten the bul­let of be­liev­ing him­self luck­y.)

I won­der if New­ton’s be­lief that he was merely re­dis­cov­er­ing what the an­cients like the Chaldeans or King Solomon knew, a con­vic­tion closely con­nected to his , is con­nected to this ‘re­cent progress’ ar­gu­ment, as it could also be taken as ev­i­dence for hu­man­ity ex­ist­ing for a long time but in con­stant cy­cles of rise and fall, im­ply­ing the pos­si­bil­ity of many cy­cles of knowl­edge be­ing es­o­ter­i­cally en­coded into the most an­cient writ­ings & ide­olo­gies?

If I have seen fur­ther it is by stand­ing on the sh­old­ers [sic] of Gi­ants.

In­ter­est­ing­ly, it seems the ‘short hu­man his­tory’ ar­gu­ment is not orig­i­nal to New­ton, and has been used else­where to de­fend the (a po­si­tion made tricky if in­deed hu­mans only came into ex­is­tence re­cent­ly) be­cause I came across a ver­sion of it in Lu­cretius’s :

More­over, if heaven and earth never had a be­gin­ning or birth, but have ex­isted from ever­last­ing, why have there not been other po­ets to sing of other events prior to the The­ban war and the tragedy of Troy? Why have so many heroic deeds so often been buried in obliv­ion, in­stead of flow­er­ing some­where, im­planted in eter­nal memo­ri­als of fame? The true ex­pla­na­tion, in my judg­ment, is that our world is in its youth: it was not cre­ated long ago, but is of com­par­a­tively re­cent ori­gin. That is why at the present time some arts are still be­ing re­fined, still be­ing de­vel­oped. This age has seen many im­prove­ments in ship­build­ing; it is not long since mu­si­cians first molded melo­di­ous tunes; our sys­tem of phi­los­o­phy too is a re­cent in­ven­tion, and I my­self am found to be the very first with the abil­ity to ex­pound it in the lan­guage of my coun­try [Lat­in].

If by chance you be­lieve that all these same things hap­pened be­fore, but that the races of hu­man be­ings per­ished in a great con­fla­gra­tion, or that their cities were razed by a mighty con­vul­sion of the world, or that rivers, ra­pa­cious after un­remit­ting rains, in­un­dated the earth and sub­merged towns, there is all the more ne­ces­sity for you to ad­mit de­feat and ac­knowl­edge that heaven and earth are des­tined to be de­stroyed.

Lu­cretius’s ar­gu­ment is not quite the same, as he equiv­o­cates a lit­tle on whether in­tel­li­gent species/hu­mans go in cy­cles of cre­ation & de­struc­tion (which ap­pears to be New­ton’s take) or if the uni­verse is sim­ply of re­cent date (which would, in­ci­den­tal­ly, be con­sis­tent with a lit­er­al­ist Bib­li­cal take in which re­cent progress is be­cause the world & hu­man­ity were cre­ated only re­cent­ly, per Gen­e­sis), but it’s the same idea: given the fact of re­cent progress in ap­par­ently easy low-hang­ing fruit like com­pos­ing epics or in­vent­ing needles, this im­plies ei­ther that hu­man­ity is it­self of re­cent ori­gin (in some sense) or that progress only be­gan re­cently for no good rea­son. But and the lat­ter is ab­surd, so they re­ject it and ac­cept the other pos­si­bil­i­ty.

An­other ar­gu­ment, de­fend­ing Lu­cretius on differ­ent grounds, is pro­vided by in his II.5 (“The Changes Of Re­li­gion And Of Lan­guages, To­gether With The Oc­cur­rence Of Del­uges And Pesti­lences, De­stroy The Record Of Things”) in which he de­fends eter­nal­ism by ar­gu­ing that we should not ex­pect any trust­wor­thy records be­cause po­lit­i­cal/re­li­gious move­ments reg­u­larly de­stroy all an­cient things (eg the ) every few thou­sand years, and then par­tic­u­larly ex­treme dis­as­ters wipe out hu­mans every few tens to hun­dreds of thou­sands of years:

To those philoso­phers who main­tain that the world has ex­isted from eter­ni­ty, we might re­ply, that, if it were re­ally of such an­tiq­ui­ty, there would rea­son­ably be some record be­yond 5000 years, were it not that we see how the records of time are de­stroyed by var­i­ous caus­es, some be­ing the acts of men and some of Heav­en. Those that are the acts of men are the changes of re­li­gion and of lan­guage; for when a new sect springs up, that is to say a new re­li­gion, the first effort is (by way of as­sert­ing it­self and gain­ing in­flu­ence) to de­stroy the old or ex­ist­ing one; and when it hap­pens that the founders of the new re­li­gion speak a differ­ent lan­guage, then the de­struc­tion of the old re­li­gion is eas­ily effect­ed. This we know from ob­serv­ing the pro­ceed­ings of the Chris­tians against the hea­then re­li­gion; for they de­stroyed all its in­sti­tu­tions and all its cer­e­monies, and effaced all record of the an­cient the­ol­o­gy. It is true that they did not suc­ceed in de­stroy­ing en­tirely the record of the glo­ri­ous deeds of the il­lus­tri­ous men of the an­cient creed, for they were forced to keep up the Latin lan­guage by the ne­ces­sity of writ­ing their new laws in that tongue; but if they could have writ­ten them in a new lan­guage (bear­ing in mind their other per­se­cu­tion­s), there would have been no record what­ever left of pre­ced­ing events. Who­ever reads the pro­ceed­ings of , and of the other heads of the Chris­t­ian re­li­gion, will see with what ob­sti­nacy they per­se­cuted all an­cient memo­ri­als, burn­ing the works of the his­to­ri­ans and of the po­ets, de­stroy­ing the stat­ues and im­ages and de­spoil­ing every­thing else that gave but an in­di­ca­tion of an­tiq­ui­ty. So that, if they had added a new lan­guage to this per­se­cu­tion, every­thing re­lat­ing to pre­vi­ous events would in a very short time have been sunk in obliv­ion.

It is rea­son­able to sup­pose that what the Chris­tians prac­tised to­wards the Pa­gans, these prac­tised in like man­ner upon their pre­de­ces­sors. And as the re­li­gions changed 2 or 3 times in 6000 years, all mem­ory of the things done be­fore that time was lost; and if nev­er­the­less some ves­tiges of it re­main, they are re­garded as fab­u­lous, and are be­lieved by no one; as is the case with the his­tory of , who gives of some 40 or 50,000 years, yet is gen­er­ally looked upon as be­ing men­da­cious, and I be­lieve with jus­tice.

As to causes pro­duced by Heav­en, they are such as de­stroy the hu­man race, and re­duce the in­hab­i­tants of some parts of the world to a very few in num­ber; such as pesti­lence, famine, or in­un­da­tions.5 Of this the lat­ter are the most im­por­tant, partly be­cause they are most uni­ver­sal, and partly be­cause the few that es­cape are chiefly ig­no­rant moun­taineers, who, hav­ing no knowl­edge of an­tiq­uity them­selves, can­not trans­mit any to pos­ter­i­ty. And should there be amongst those who es­cape any that have such knowl­edge, they con­ceal or per­vert it in their own fash­ion, for the pur­pose of gain­ing in­flu­ence and rep­u­ta­tion; so that there re­mains to their suc­ces­sors only just so much as they were dis­posed to write, and no more. And that such in­un­da­tions, pesti­lences, and famines oc­cur can­not be doubt­ed, both be­cause all his­tory is full of ac­counts of them, and be­cause we see the effects of them in the obliv­ion of things, and also be­cause it seems rea­son­able that they should oc­cur.

This is a rea­son­able ar­gu­ment by Machi­avel­li: it is true that re­li­gions at­tempt to erase and re­cy­cle their pre­de­ces­sors and if only through ne­glect, as­sure their dis­ap­pear­ance6, and it is also true that oc­ca­sional cat­a­stro­phes like pan­demics can de­pop­u­late re­gions & end civ­i­liza­tions (eg the New World, or the Black Plague). Yet, he is still wrong. There were no civ­i­liza­tions 50,000 years ago, and more than enough sur­vives of pre-Chris­t­ian or pre-Is­lamic civ­i­liza­tions for us to un­der­stand them.

The claims of and evo­lu­tion—that the ob­served ge­og­ra­phy and bi­ol­ogy arose only in­cre­men­tally over hy­poth­e­sized pe­ri­ods of mil­lions or even bil­lions of years, such lengths be­ing nec­es­sary to ac­count for the hu­man-ob­served short­-term sta­sis of species or ge­og­ra­phy—­would have doubt­less struck them as even more ab­surd. Who could ac­cept a the­ory like “after bil­lions of years, hu­mans evolved, but then noth­ing of any im­por­tance hap­pened for hun­dreds of thou­sands or mil­lions of years, un­til then every­thing started hap­pen­ing in the past 10,000 years at an ever-ac­cel­er­at­ing pace”? Nev­er­the­less… (Here we have a sit­u­a­tion sim­i­lar to that of , of be­ing wrong for the right rea­sons. notes the irony: par­tic­u­larly in the Re­nais­sance, these the­o­ries of an eter­nal world emerged as part of the awak­en­ing of crit­i­cal thought about the­o­log­i­cal dog­mas like Cre­ation­is­m—but it was the dog­mas which were right about the uni­verse be­ing of a fi­nite age.)

From our per­spec­tive, the pace of progress in New­ton’s day—n­ever mind Lu­cretius 1700 years pri­or—was ag­o­niz­ingly slow and near-in­vis­i­ble, but to New­ton and his con­tem­po­raries, it must have ap­peared rapid, even more rapid than for Lu­cretius.

This offers a lit­tle twist on the “Sin­gu­lar­ity” idea: ap­par­ently peo­ple have al­ways been able to see progress as rapid in the right time pe­ri­ods, and they are not wrong to! We would not be too im­pressed at sev­eral cen­turies with merely some ship­build­ing im­prove­ments or a long phi­los­o­phy poem writ­ten in Lat­in, and we are only mod­estly im­pressed by nee­dles or print­ing press­es. Hu­man his­to­ry, start­ing some­time in the Ne­olithic, is a gi­ant ex­po­nen­tial, which just keeps go­ing up. What seemed star­tlingly rapid at one point may seem ag­o­niz­ingly slow viewed from a later stand­point.

  1. How this can be rec­on­ciled with his be­liefs in a Sec­ond Com­ing of Je­sus or the var­i­ous or even the oc­cur­rence of an Apoc­a­lypse & Sec­ond Com­ing of Je­sus, I do not know.↩︎

  2. This is a con­fus­ing sen­tence and the scan is not avail­able to check, but I in­ter­pret Con­duitt as mean­ing “that all arts, such as [al­pha­bet­ic] let­ters, long [o­cean-go­ing] ships, print­ing [press­es], nee­dles [and thread], etc”.↩︎

  3. New­ton was not the only as­tronomer to sus­pect in­tel­li­gent life on the Jov­ian moons. In April 1610, wrote to Galileo of his :

    …Mean­while I can­not re­frain from con­tribut­ing this ad­di­tional fea­ture to the un­ortho­dox as­pects of your find­ings. It is not im­prob­a­ble, I must point out,344 that there are in­hab­i­tants not only on the moon but on Jupiter too or (as was de­light­fully re­marked at a re­cent gath­er­ing of cer­tain philoso­phers345) that those ar­eas are now be­ing un­veiled for the first time. But as soon as some­body demon­strates the art of fly­ing, set­tlers from our species of man will not be lack­ing. Who would once have thought that the cross­ing of the wide ocean was calmer and safer346 than of the nar­row Adri­atic Sea, Baltic Sea, or Eng­lish Chan­nel? Given ships or sails adapted to the breezes of heav­en, there will be those who will not shrink from even that vast ex­panse. There­fore, for the sake of those who, as it were, will presently be on hand to at­tempt this voy­age, let us es­tab­lish the as­tron­o­my, Galileo, you of Jupiter, and me of the moon.

    1. This state­ment was mis­tak­enly trans­ferred from Ke­pler to Galileo by Bryk (p. 347). Galileo did not at­tribute in­hab­i­tants to the moon and to Jupiter, de­spite Bryk.
    2. In the let­ter of April 19 (NE X, 336:606) Ke­pler said in­stead: “at Wack­her’s din­ner ta­ble” (in mensa nos­tri Vack­herii).
    3. In “Four Voy­ages” the first two cross­ings were calm and safe. Per­haps Ke­pler did not read as far as the third voy­age (“On those days [of the At­lantic cross­ing] we ex­pe­ri­enced worse weather than any­one had ever suffered at sea be­fore”) and the fourth voy­age (“A fierce and bit­ter storm arose, and a con­trary wind and ad­verse weather pre­vailed”); in , fol. e2v, Bv.
  4. Pg189–1990, chap­ter 41, trans. Stoner 1930:

    Among the ex­tra­or­di­nary, though quite nat­ural cir­cum­stances of my life, the first and most un­usual is that I was born in this cen­tury in which the whole world be­came known; whereas the an­cients were fa­mil­iar with but lit­tle more than a third part of it. On the one hand, we ex­plore amer­i­ca…Be­sides all the­se, to­wards the East un­der the Antarc­tic…as well as Japan, Bi­na­rchi­a…all dis­cov­er­ies sure to give rise to great and calami­tous events in or­der that a just dis­tri­b­u­tion of them may be main­tained.

    …[M]ean­while we shall re­joice as in a flow­er-filled mead­ow. For what is more amaz­ing than py­rotech­nics? Or than the fiery bolts man has in­vented so much more de­struc­tive than the light­ning of the gods?

    Nor of thee, O Great Com­pass, will I be silent, for thou dost guide us over bound­less seas, through gloomy nights, through the wild storms sea­far­ers dread, and through the path­less wilder­ness.

    The fourth mar­vel is the in­ven­tion of the ty­po­graphic art, a work of man’s hands, and the dis­cov­ery of his wit—a ri­val, for­sooth, of the won­ders wrought by di­vine in­tel­li­gence. What lack we yet un­less it be the tak­ing of Heaven by storm! Oh, the mad­ness of men to give heed to van­ity rather than the fun­da­men­tal things of life! Oh, what ar­ro­gant poverty of in­tel­lec­tual hu­mil­ity not to be moved to won­der! But to re­turn to my theme [con­cern­ing the aveng­ing of my son]…

  5. Note Machi­avel­li’s here.↩︎

  6. As al­most hap­pened to Lu­cretius (2 or 3 sur­viv­ing copies) and (only a quar­ter sur­vives); and as Machi­avelli notes later on, the Ro­mans were un­kind to their pre­de­ces­sors, who re­main mys­te­ri­ous. (We could also note that mod­ern pop­u­la­tion ge­net­ics has shown that near-to­tal pop­u­la­tion re­place­ment is quite com­mon through­out deep his­to­ry.)↩︎