If decapod does not pass through zoea stage, is this acceleration? If hypothetical adult retained zoea characters, would this be retardation? Believes obliteration of growth stages frequently due to natural selection. Most interesting points in AH’s letter deal with senile characters. CD attributes them to laws of growth not selection. Explains degraded characters as result of readaptation to simpler conditions. Believes no innate tendency to progressive development exists.
Hopes AH visits F. Hilgendorf’s famous deposit [at Steinheim]. A. Weismann [Einfluss der Isolierung (1872)] makes good use of Hilgendorf’s observations.
Down, | Beckenham, Kent.
My dear Sir
I thank you sincerely for yr most interesting letter.f2 You refer muchtoo modestly to yr own knowledge & judgement, as you are much betterfitted to throw light on your own difficult problems than I am. It hasquite annoyed me that I cd not clearly understand your Prof. Cope’sviews; & the fault lies in some slight degree, I think, withProf. Cope, who does not write very clearly.f3 I think I now understandthe terms “acceleration & retardation”; but will you grudge thetrouble of telling me, by the aid of the following illustration,whether I do understand rightly? When a fresh-water, decapodcrustacean is born with an almost mature structure, & therefore doesnot pass, like other decapods, though the Zoea stage,f4 is this not acase of acceleration? Again, if an imaginary decapod retained whenadult many Zoea characters, wd this not be a case of retardation? Ifthese illustrations are correct I can perceive why I have been so dullin understanding your views: I looked for something else, beingfamiliar with such cases, & classing them in my own mind as simply dueto the obliteration of certain larval or embryonic stages. Thisobliteration I imagined resulted somestimes entirely from that law ofinheritance to which you allude;f5 but that in many cases it was aidedby natural selection, as I inferred from such cases occurring sofrequently in terrestrial & fresh-water members of groups, whichretain their several embryonic stages in the sea, as long as fittingconditions are present.
Another cause of my misunderstanding was the assumption that inyour series
the differences between the successive species, expressed by theterminal letters, were due to “acceleration”: now if I understandrightly, this is not the case; & such characters must have beenindependently acquired by some means.f6
The two newest & most interesting points in your letter (& in asfar as I remember your former paper) seem to me to be about senilecharacters in one species appearing in succeeding species, duringmaturity; & secondly about certain degraded characters appearing inthe last species of a series.— You ask for my opinion; but I cansend only the conjectured impressions which have occurred to me, &which are at most worth nothing. (It ought to be known whether thesenile characters appear before or after the period of activereproduction) I shd be inclined to attribute the characters in bothyour cases to the laws of growth & quite secondarily to naturalselection.—f7 It has been an error on my part & a misfortune to me,that I did not largely discuss what I mean by laws of growth at anearly period in some of my books. I have said something on this headin the new Chapt. in the last Edit. of the Origin.f8 I shd. be happyto send you a copy of this Edit, if you do not possess it & care tohave it. A man in extreme old age differs much from a young man, & Ipresume every one would account for this by failing powers ofgrowth. On the other hand the skulls of some mammals go on alteringduring maturity with advancing years,—as do the horns of stags, thetail-feathers of some birds, the size of fishes &c; & all suchdifferences I shd attribute simply to the laws of growth, as long asfull vigour was retained. Endless other changes of structure insuccessive species may I believe be accounted for by various complexlaws of growth. Now any change of character thus induced withadvancing years in the individual might easily be inherited at anearlier age than that at which it first supervened, & thus becomecharacteristic of the mature species; or again, such changes wd beapt to follow from variation, independently of inheritance, underproper conditions. Therefore I shd expect that characters of thiskind wd often appear in later-formed species without the aid ofnatural selection, or with its aid if the characters were of anyadvantage.
The longer I live the more I become convinced how ignorant we are ofthe extent to which all sorts of structures are serviceable to eachspecies. But that characters supervening during maturity in onespecies shd appear so regularly, as you state to be the case, insucceeding species seems to me very surprising & inexplicable.
With respect to degradation in species towards the close of aseries, I have nothing to say, except that before I arrived at the endof yr letter, it occurred to me that the earlier & simplerammonites must have been well adapted to their conditions, & that whenthe species were verging towards extinction (owing probably to 〈th〉epresence of some more successful competitors) they wd naturallybecome readapted to simpler conditions. Before I had read yr finalremarks I thought also that unfavourable conditions might cause,thro’ the laws of growth, aided perhaps by reversion, degradation ofcharacter.f9 No doubt many new laws remain to be discovered. Permit meto add that I have never been so foolish as to imagine that I havesucceeded in doing more than to lay down some of the broad outlines ofthe origin of species. After long reflection I cannot avoid theconviction that no innate tendency to progressive development exists,as is now held by so many able naturalists, & perhaps by yourself. Itis curious how seldom writers define what they mean by progressivedevelopment; but this is a point which I have briefly discussed in theOrigin.f10
I earnestly hope that you may visit Hilgendorf’s famousdeposit.f11 Have you seen Weismann’s pamphlet “Einfluss der Isolirung”Leipzig 1872 He makes splendid use of Hilgendorf’s admirableobservations.f12 I have no strength to spare, being much out of health;otherwise I wd have endeavoured to have made this letter betterworth sending.
I most sincerely wish you success in yr valuable & difficultresearches & I remain | my dear Sir | yours very faithfully |Ch. Darwin
P.S I have received & thank you for yr 3 pamphlets.f13
As far as I can judge, yr views seem very probable; but what afearfully intricate subject is this of the succession of Ammonites.