To Alpheus Hyatt 4 December 1
Down, | Beckenham, Kent.
My dear Sir
I thank you sincerely for yr most interesting letter.2 You refer much too modestly to yr own knowledge & judgement, as you are much better fitted to throw light on your own difficult problems than I am. It has quite annoyed me that I cd not clearly understand your Prof. Cope’s views; & the fault lies in some slight degree, I think, with Prof. Cope, who does not write very clearly.3 I think I now understand the terms “acceleration & retardation”; but will you grudge the trouble of telling me, by the aid of the following illustration, whether I do understand rightly? When a fresh-water, decapod crustacean is born with an almost mature structure, & therefore does not pass, like other decapods, though the Zoea stage,4 is this not a case of acceleration? Again, if an imaginary decapod retained when adult many Zoea characters, wd this not be a case of retardation? If these illustrations are correct I can perceive why I have been so dull in understanding your views: I looked for something else, being familiar with such cases, & classing them in my own mind as simply due to the obliteration of certain larval or embryonic stages. This obliteration I imagined resulted somestimes entirely from that law of inheritance to which you allude;5 but that in many cases it was aided by natural selection, as I inferred from such cases occurring so frequently in terrestrial & fresh-water members of groups, which retain their several embryonic stages in the sea, as long as fitting conditions are present.
Another cause of my misunderstanding was the assumption that in your series
the differences between the successive species, expressed by the terminal letters, were due to “acceleration”: now if I understand rightly, this is not the case; & such characters must have been independently acquired by some means.6
The two newest & most interesting points in your letter (& in as far as I remember your former paper) seem to me to be about senile characters in one species appearing in succeeding species, during maturity; & secondly about certain degraded characters appearing in the last species of a series.— You ask for my opinion; but I can send only the conjectured impressions which have occurred to me, & which are at most worth nothing. (It ought to be known whether the senile characters appear before or after the period of active reproduction) I shd be inclined to attribute the characters in both your cases to the laws of growth & quite secondarily to natural selection.—7 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 an early period in some of my books. I have said something on this head in the new Chapt. in the last Edit. of the Origin.8 I shd. be happy to send you a copy of this Edit, if you do not possess it & care to have it. A man in extreme old age differs much from a young man, & I presume every one would account for this by failing powers of growth. On the other hand the skulls of some mammals go on altering during maturity with advancing years,—as do the horns of stags, the tail-feathers of some birds, the size of fishes &c; & all such differences I shd attribute simply to the laws of growth, as long as full vigour was retained. Endless other changes of structure in successive species may I believe be accounted for by various complex laws of growth. Now any change of character thus induced with advancing years in the individual might easily be inherited at an earlier age than that at which it first supervened, & thus become characteristic of the mature species; or again, such changes wd be apt to follow from variation, independently of inheritance, under proper conditions. Therefore I shd expect that characters of this kind wd often appear in later-formed species without the aid of natural selection, or with its aid if the characters were of any advantage.
The longer I live the more I become convinced how ignorant we are of the extent to which all sorts of structures are serviceable to each species. But that characters supervening during maturity in one species shd appear so regularly, as you state to be the case, in succeeding species seems to me very surprising & inexplicable.
With respect to degradation in species towards the close of a series, I have nothing to say, except that before I arrived at the end of yr letter, it occurred to me that the earlier & simpler ammonites must have been well adapted to their conditions, & that when the species were verging towards extinction (owing probably to 〈th〉e presence of some more successful competitors) they wd naturally become readapted to simpler conditions. Before I had read yr final remarks I thought also that unfavourable conditions might cause, thro’ the laws of growth, aided perhaps by reversion, degradation of character.9 No doubt many new laws remain to be discovered. Permit me to add that I have never been so foolish as to imagine that I have succeeded in doing more than to lay down some of the broad outlines of the origin of species. After long reflection I cannot avoid the conviction that no innate tendency to progressive development exists, as is now held by so many able naturalists, & perhaps by yourself. It is curious how seldom writers define what they mean by progressive development; but this is a point which I have briefly discussed in the Origin.10
I earnestly hope that you may visit Hilgendorf’s famous deposit.11 Have you seen Weismann’s pamphlet “Einfluss der Isolirung” Leipzig 1872 He makes splendid use of Hilgendorf’s admirable observations.12 I have no strength to spare, being much out of health; otherwise I wd have endeavoured to have made this letter better worth sending.
I most sincerely wish you success in yr valuable & difficult researches & I remain | my dear Sir | yours very faithfully | Ch. Darwin
P.S I have received & thank you for yr 3 pamphlets.13
As far as I can judge, yr views seem very probable; but what a fearfully intricate subject is this of the succession of Ammonites.
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.
Please cite as
Darwin Correspondence Project, “Letter no. 8658,” accessed on 28 February 2017, http://www.darwinproject.ac.uk/DCP-LETT-8658