I thank you sincerely for your most interesting letter. You refer much too modestly to your own knowledge and judgment, as you are much better fitted to throw light on your own difficult problems than I am.
It has quite annoyed me that I do not clearly understand yours and Prof. Cope's views (254/1. Prof. Cope's views may be gathered from his "Origin of the Fittest" 1887; in this book (page 41) is reprinted his "Origin of Genera" from the "Proc. Philadelph. Acad. Nat. Soc." 1868, which was published separately by the author in 1869, and which we believe to be his first publication on the subject. In the preface to the "Origin of the Fittest," page vi, he sums up the chief points in the "Origin of Genera" under seven heads, of which the following are the most important:--"First, that development of new characters has been accomplished by an ACCELERATION or RETARDATION in the growth of the parts changed...Second, that of EXACT PARALLELISM between the adult of one individual or set of individuals, and a transitional stage of one or more other individuals. This doctrine is distinct from that of an exact parallelism, which had already been stated by von Baer." The last point is less definitely stated by Hyatt in his letter of December 4th, 1872. "I am thus perpetually led to look upon a series very much as upon an individual, and think that I have found that in many instances these afford parallel changes." See also "Lamarck the Founder of Evolution, by A.S. Packard: New York, 1901.) and the fault lies in some slight degree, I think, with Prof. Cope, who does not write very clearly. I think I now understand the terms "acceleration" and "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, and therefore does not pass, like other decapods, through the Zoea stage, is this not a case of acceleration? Again, if an imaginary decapod retained, when adult, many Zoea characters, would 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, and classing them in my own mind as simply due to the obliteration of certain larval or embryonic stages. This obliteration I imagined resulted sometimes entirely from that law of inheritance to which you allude; but that it in many cases was aided by Natural Selection, as I inferred from such cases occurring so frequently in terrestrial and 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 letter, was due to acceleration: now, if I understand rightly, this is not the case; and such characters must have been independently acquired by some means.
The two newest and most interesting points in your letter (and in, as far as I think, your former paper) seem to me to be about senile characteristics in one species appearing in succeeding species during maturity; and secondly about certain degraded characters appearing in the last species of a series. You ask for my opinion: I can only send the conjectured impressions which have occurred to me and which are not worth writing. (It ought to be known whether the senile character appears before or after the period of active reproduction.) I should be inclined to attribute the character in both your cases to the laws of growth and descent, secondarily to Natural Selection. It has been an error on my part, and 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 two new chapters in the last edition of the "Origin." I should be happy to send you a copy of this edition, if you do not possess it and care to have it. A man in extreme old age differs much from a young man, and 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 into advancing years; as do the horns of the stag, the tail-feathers of some birds, the size of fishes etc.; and all such differences I should 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, and thus become characteristic of the mature species; or again, such changes would be apt to follow from variation, independently of inheritance, under proper conditions. Therefore I should expect that characters of this kind would 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 should appear so regularly, as you state to be the case, in succeeding species, seems to me very surprising and 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 your letter, it occurred to me that the earlier and simpler ammonites must have been well adapted to their conditions, and that when the species were verging towards extinction (owing probably to the presence of some more successful competitors) they would naturally become re-adapted to simpler conditions. Before I had read your final remarks I thought also that unfavourable conditions might cause, through the law of growth, aided perhaps by reversion, degradation of character. 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, and 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." I earnestly hope that you may visit Hilgendorf's famous deposit. Have you seen Weismann's pamphlet "Einfluss der Isolirung," Leipzig, 1872? He makes splendid use of Hilgendorf's admirable observations. (254/2. Hilgendorf, "Monatsb. K. Akad." Berlin, 1866. For a semi-popular account of Hilgendorf's and Hyatt's work on this subject, see Romanes' "Darwin and after Darwin," I., page 201.) I have no strength to spare, being much out of health; otherwise I would have endeavoured to have made this letter better worth sending. I most sincerely wish you success in your valuable and difficult researches.
I have received, and thank you, for your three pamphlets. As far as I can judge, your views seem very probable; but what a fearfully intricate subject is this of the succession of ammonites. (254/3. See various papers in the publications of the "Boston Soc. Nat. Hist." and in the "Bulletin of the Harvard Museum of Comp. Zoology.")